Get our free book (in Spanish or English) on rainwater now - To Catch the Rain.

Solar Domestic Water Heating

From Appropedia
Jump to navigation Jump to search

Contents

Introduction[edit]

Currently fossil fuels provide more than 85% of the world primary energy demand. As world population increases, global energy demand can only increase. The reality of today’s energy crisis demands that increases in fuel burning efficiency are not sufficient to arrive at a sustainable level of fossil fuel consumption. These realities, coupled with increased difficulty and cost of future fossil fuel extraction, make conservation of fossil fuels a major concern.

It is possible to cover the entire global energy demand using renewable sources, but this aspiration requires a completely new energy infrastructure. Additionally, many of these sources fluctuate, thus any renewable energy system must be capable of generating useful energy and to guarantee consistent availability.

Solar water heating offers a renewable energy technology opportunity which could be globally implemented, requires little behavior changes to the end user, and can ultimately reduce fossil fuel consumption. Solar thermal systems can be applied to space heating, water heating, industrial processes, cooling applications or electricity generation.

Background of Solar Technologies[edit]

The sun is largest energy resource on earth; approximately 3.9x1024 J of energy reaches earth’s surface each year, which could cover around 10,000 times the global energy requirements [28]. This abundant source of energy has led to the development of a variety of technical systems to convert solar radiation into a useful form of energy for society, including photovoltaics, solar power plants, and photolysis systems. Indirectly, solar energy creates wind, river current and plant growth, whose energy can also be exploited for use to meet the global energy demand.

Interestingly, the technology for a variety of renewable energies is already available. Historically engineers and scientists developed technologies to increase the quality of life before electricity was implemented into widespread use at the end of the 18th century. These technologies include the use of firewood, wind and water turbines or other alternative technologies.

Today the most widespread use of solar energy is for water heating. Hot water is a fundamental necessity in domestic, commercial and industrial sectors, and can account for 15-25% of energy consumed in homes. Industrially, water heating may account for even more energy. The application of solar energy for domestic water heating rather than space heating is especially appealing due to its consistent need year round.

Background of SDHW Systems[edit]

Solar water heating is particularly appealing because of its mature development stage. It is the only solar application that had a commercial existence before federal solar energy programs in the early 70’s. Because of the quick commercialization of this technology, federal funding of these systems focuses on development of test methods, evaluation and demonstration of new systems rather than design research. Development in solar water heating has largely been completed by the solar industry, making these systems more easily installed than other more complicated technologies.

The first commercial solar water heaters were installed in southern California in the late 1800’s, first as roof-mounted blackened tanks with no insulation and later in conjunction with a wood stove to provide a continuous source of hot water. Later designs used glazed tubular solar collectors separated from elevated storage tanks in thermosyphon arrangement, a configuration which is still used in mild climates. Auxiliary heaters were often installed in these systems to ensure a continuous hot water supply. Later, heat exchangers were also implemented into these systems as a method of freeze protection. California use of solar water heaters declined in the 1920’s and 30’s due to increased availability of natural gas and low electricity prices.

Around the same time, population increases in the Miami area prompted increased use of solar water heating systems; it is estimated that over 50,000 systems were manufactured and sold in this time period. The most common system used in the Florida was the “SunCoil,” which consisted of a copper absorber with insulated housing connected to residential plumbing. These systems flourished until the early 1940’s when the government instituted a freeze on non-military use of copper.

Following World War II, the introduction of washing machines and dishwashers further increased the domestic hot water demand beyond what a solar water heating system could provide. These factors, along with the introduction of mass-produced electric water heaters and low fuel costs contributed to the decline of solar water heaters in the US in the 1950’s.

While the US saw a decline in the application of solar water heating around the middle of the 20th century, international increases in electricity prices spurred development of new solar water heating designs around the world. Government policy in Israel in the 1950’s required the installation of thermosyphon solar water heaters in new housing projects. A similar level of wide-spread commercialization of solar water heaters was achieved in Australia and Japan in the 1970’s.

In the United States, a renewed increase in the manufacture and sale of solar water heaters began after the oil embargo of 1973. Many companies began manufacture of flat-plate solar collectors, and by 1979 solar water heating systems had were being installed at a rate of nearly 50,000 systems per year, a number which increased two or three-fold by 1984 (DOE).

The past decade has shown significant increase in the use of solar domestic water heating systems due to the global pressures to reduce greenhouse gas emissions as well as the quest for sustainable energy solutions. Today, solar water heating is most widely practiced solar technology; an estimated 500,000 – 800,000 residential solar water heaters are installed in the US today. While these numbers are encouraging, less than 1% of residential and commercial establishments in the United States are provided with solar heated water. This, compared to an approximated 10 million systems installed in China and high installation rates in Europe, India, Egypt and Turkey, leaves a wide berth for improvement. Current systems are reliable as a primary water heating method, however widespread use is limited by market demand, which will not likely improve until conventional fuel prices increase significantly.

Current Technologies[edit]

The most basic solar water heating system consists of a tank filled with water that is exposed to sunlight in warm temperatures. The water inside the tank is heated up and can be connected to residential plumbing. While this set-up is useful to understand a basic solar water heating system, it is too simplistic for realistic use. More complex systems have been designed which separate the collector from the thermal storage tank.

Solar domestic hot water systems could reduce total household energy use by 50% [10]. Currently, around 100MW of solar water heating systems have been installed in the United States. Comparing this value to installation rates in Europe (2000MW) and China (15000MW), it is evident that the solar water heating market in the US is not yet competitive with the rest of the world. System costs in the United States are limited by lack of retailers and a knowledgeable service sector. Total life cycle costs are narrowly cost-effective compared to the cost of a traditional water heating system.

System Components[edit]

Every solar domestic water heating system consists of the same basic equipment. A solar collector collects solar radiation and concentrates the thermal energy to a thermal reservoir. Piping and other structural equipment are also required for system functionality. More complicated designs may include circulation pumps, sensing and controls, heat exchangers with a recirculating thermal fluid, auxiliary heating capabilities or other sensor-regulated capabilities.

Collectors[edit]

The purpose of the collector to collect, concentrate and convert solar radiation to useful thermal energy. An absorber converts sunlight to heat which is then transferred to the domestic water supply or to a separate recirculating thermal fluid. Absorber material is typically metallic, such as copper, steel or aluminum, which is then coated to improve heat absorption. This can be accomplished through the use of black lacquer. The use of more efficient other selective coatings can help reduce losses at an increased cost.

The absorber is protected by a transparent cover to minimize convective and radiative losses to the environment and to serve as a protective barrier. The covering of the collector can be described in terms of reflectance ρ, absorbance α and transmittance τ. Generally, low reflectance and high absorbance and transmittance glass or plastic is desirable for highly efficient collectors. Low-iron solar glass is often used because of its high transmittance and durability to environmental hazards.

The absorber is contained in a housing which is highly insulated on the back and sides to minimize heat loss. The housing and binding materials must be temperature-resistant and the glass plate must be properly sealed to the collector housing to contain heat and prevent dirt and humidity from entering the chamber.

To properly understand collector performance, conversion efficiency from solar radiation to useful heat is considered. Solar irradiance E is converted to heat Q through the transparent covering with transmittance τ onto a collector area Ac. Overall conversion in limited by reflection ρ and convection to the atmosphere. Overall collected heat is given as:

Equation 1

Generally radiative losses are minimal compared to reflective and convective losses and can be neglected. The implementation of a vacuum in the collector can minimize convective losses.

Reflective losses can be quantified by knowing the reflectance from the radiation passing through the glass covering. Maximum efficiency for a particular system is achieved by eliminating convective and radiative losses, and is thus a function of reflective losses only. Maximum collector efficiency is given by:

Equation 2

Taking into account convective and radiative losses, which largely depend on temperature difference between the collector interior and the ambient environment, yield actual collector efficiency as:

Equation 3

Other environmental effects can be included in the analysis above as loss coefficients, and will account for additional losses.

Here, it is important to note that collector efficiency is a function of cover properties, collector area, temperature difference, and solar insolation. A greater temperature range between the absorber and the environment increases cycle efficiency, but also increases thermal losses. As a result, overall temperature range must be monitored to ensure appropriate process efficiency, and collector materials must be able to withstand a substantial temperature range.

The most common types of collectors for residential solar water heating are flat-plate collectors, evacuated flat-plate collectors and evacuated-tube collectors.

Flat-plate Collector[edit]

Figure 1. Schematic of Flat-Plate Collector Processes [28]

A flat-plate collector is the most simple collector design, characterized by a flat absorber plate over metal tubes containing the domestic water supply or a thermal fluid as found in Figure 1.

Despite lower conversion efficiencies than other collector designs, the appeal of flat-plate collectors lies in the simplicity of the design. The major disadvantage to flat-plate collectors is the requirement of a storage tank due to their small size; these collectors can quickly overheat if the water inside is not continuously replaced.



Evacuated Flat-plate Collector[edit]

An evacuated flat-plate collector is an improved version of the standard flat-plate collector, which minimizes convective losses by implementing a vacuum in the collector chamber. Atmospheric air pressure on the glass plate necessitates the installation of supports between the back of the collector and the front cover to keep the unit from collapsing. The vacuum requires occasional maintenance as the seals on the system are subject to some losses.

Evacuated Tube Collector[edit]

Figure 2. Schematic of Evacuated Tube Collector Function [28]

Evacuated tube collectors further develop the concept of an evacuated flat-plate collector by embedding a metal absorber sheet with a heat pipe in the middle in an evacuated glass tube. A temperature-sensitive thermal fluid is heated and allowed to vaporize inside the tube. The fluid rises to a heat exchanger and condenser system to capture the heat from the medium, then falls back to the bottom of the tube when it can be re-vaporized. This type of system depends on geometry for proper functionality, thus the inclination angle of evacuated tube collectors is particularly important.

Evacuated tube collectors are especially appealing in that the vacuum inside a glass tube is easier to maintain than that of a flat-plate; their geometry allows them to resist ambient air pressure, eliminating the need for additional supports. Evacuated tube collectors can more effectively capture heat from solar radiation requiring a smaller collector area than comparable flat-plate collectors.

The major limitation to evacuated tube collectors is increased cost. Materials and construction costs are higher than for other collector types, and the system as a whole may require additional support equipment. For example small molecules such as atmospheric hydrogen can enter the system, eventually eliminating the vacuum. This can be remedied by installing “getters” to absorb hydrogen over time, or by using a vacuum pump to replenish the vacuum from time to time which add to collector cost.

Example: Roof-integrated Collector [17][edit]

One new collector design is presented by Luis Juanicó of the Bariloche Atomic Center in Bariloche, Argentina. The system takes advantages of the relationship between roof-mounted collectors and the roof itself. The concept is based on expanding collector area to the entire roof, allowing for the additional potential for space heating to a water heating system.

The proposed system is an evolved version of the SkyTherm, originally proposed by Harold Hay in 1977. The SkyTherm consisted of water bags mounted over a simple metallic roof, which were protected by a folding insulated cover. The cover served to collect and store solar energy in the winter, and perform the opposite in the summer. Generally, the SkyTherm is considered to be the foundation of all roof collector designs that exist today.

Today, most roof collectors consist of a metallic roof composed of corrugated or trapezoidal metal sheets which serve as the absorber and contain piping with domestic water. Recent window glazing and piping technologies have helped to reduce precipitation leaks of these systems, however the high cost to achieve adequate thermal insulation limits the appeal of these systems.

The system proposed by Juanicó uses a metallic waterproof base with a cover of double-layer glass. A shallow water pond is encased between the metal base and the glass, and is connected with piping to a thermal storage tank. Additional components may include pumps, auxiliary heaters or a rolling awning (such as that proposed by the SkyTherm) to provide an additional insulation chamber. This system can be applied to an inclined roof simply by ensuring the main water chamber is completely airtight.

The proposed system functions by collecting solar thermal energy during the day and storing it during the night; the insulating cover can be closed at night to reduce thermal losses. Ideally, the thermal energy available from an extended collector surface area is plenty sufficient for domestic hot water, and may also be applicable to space heating and cooling.

The proposed roof-integrated collector is that it eliminates the need for a thick insulation layer due to the sufficient insulating properties of the water pond. Temperature distribution throughout the collector water is gradual, resulting in more evenly-distributed thermal properties of the water. Because this system was proposed from a roof standpoint rather than a collector standpoint, it is applicable to a variety of climates and roof configurations.

Thermal Storage[edit]

Some form of thermal storage is required for all solar water heating systems. This can be integrated with the collector, however most collectors are too small to store enough water to meet the hot water demand of a household. Thermal storage tanks must have the physical and thermal capacity to store sufficient water to meet domestic hot water demands over a designated period of time, and are insulated to retain as much heat as possible when not in use.

Thermal storage systems can be installed as single or multiple tank configurations; generally, single tank systems are more efficient than multiple tank systems as long as the unit is stratified and well-insulated. Tank stratification is used to reduce collector inlet temperature, which increases thermal efficiency of the collector. Ultimately, appropriate thermal storage configuration is determined by application, load demand and available solar resource to meet necessary hot water requirements of a particular site.

In general, storage volume should be 1.5-2 times the daily hot water demand to account for times of limited solar radiation. Insulation is an important consideration to limit heat losses; best practice entails insulation with a thickness of at least 100mm and thermal conductivity of 0.04W/mK or lower [28]. Because heat loss to the environment is highly proportional to residence time in the tank, it is important to consider how long heat must be maintained in the unit. A high surface area to volume ratio of the tank can further contribute to thermal losses.

Mathematical calculations can be used to estimate thermal storage capacities, however accurate results may be difficult to achieve due to constant heat addition and subtraction; complex computer algorithms are the only method to accurately estimate variation of water temperature inside a storage unit. For simplicity, calculations here are limited to the assumption of an average water temperature throughout the tank rather than accounting for tank stratification. When necessary, tank stratification can be accounted for by dividing the tank into layers of varying heat. The simplified heat storage capacity of a thermal storage tank is then given by:

Equation 4

Thermal losses are calculated by the following

Equation 5

It is important to note that deviation between optimal and actual storage temperatures can affect the running characteristics of a water heating system. If storage temperature is greater than desired load temperature, an appropriate quantity of hot water from the storage tank must be discharged and mixed with cold water to arrive at desired load temperature and flow rate. If storage temperature is the below the desired load temperature, additional heat requirements must be met by an auxiliary heater. In all cases, the cold water inlet is at the bottom of the tank and hot water exits the system at the top of the tank.

Piping[edit]

Collector flow rate is influenced by pipe geometry; pipe diameter can be found knowing the heating requirements from the collector and taking into account associated thermal losses. Flow rate calculated as a function of required heat is given by:

Equation 6

Required pipe diameter can be calculated by:

Equation 7

Thermal losses from piping, including circulation and heat-up losses, should be considered when evaluating a solar water heating system. Circulation losses refer to heat-loss to the environment as a function of total piping length and time, governed by:

Equation 8

Heat-up losses result from the energy required to heat up the pipes and the heat transfer fluid after they have cooled when the system is out of service. These losses can also occur at stop valves, pumps and other required piping components, and can be described for n-cycles by:

Equation 9

Additional Equipment[edit]

There is a variety of additional equipment that can be included in a basic solar water heating system to improve performance. These include pumps, controls and sensors, heat exchangers, and auxiliary heat sources, as well as a multitude of other options.

Pumps and Blowers[edit]

Pumps and blowers can be installed where there is an auxiliary power source to improve system circulation and eliminate the need for a height differential between the collector and thermal storage units. Recirculating pumps can also be used as a method of freeze protection. While the installation of pumps and blowers require additional power, system efficiency can be significantly improved and the system can potentially be installed in regions where otherwise impossible.

Controls and Sensors[edit]

Pumps and blowers can be installed where there is an auxiliary power source to improve system circulation and eliminate the need for a height differential between the collector and thermal storage units. Recirculating pumps can also be used as a method of freeze protection. While the installation of pumps and blowers require additional power, system efficiency can be significantly improved and the system can potentially be installed in regions where otherwise impossible.

Heat Exchanger[edit]

Heat exchangers may be installed for indirect solar water heating to prevent freezing in cold regions and improve thermal efficiency. Generally, a thermal fluid in a closed loop is heated in the collector, and heat is transferred to a potable water source for domestic use. While these systems may cost more and require extra maintenance, their versatility make them appealing, particularly in regions where freezing is a concern.

Auxiliary Heat Source[edit]

Heat exchangers may be installed for indirect solar water heating to prevent freezing in cold regions and improve thermal efficiency. Generally, a thermal fluid in a closed loop is heated in the collector, and heat is transferred to a potable water source for domestic use. While these systems may cost more and require extra maintenance, their versatility makes them appealing, particularly in regions where freezing is a concern.

Solar Water Heating[edit]

The basic concept behind solar thermal water heating is the transfer of heat from solar radiation to a thermal fluid. In cases where the thermal fluid is not the domestic water source, heat is transferred from the thermal fluid to the domestic water source with a heat exchanger. Thermal storage is used to store the heat until it is supplied to the user. Sensing and pumping capabilities as well as the use of an auxiliary heat source improve system accuracy and ensure sufficient hot water delivery to the residence.

Solar water heating systems can be divided into passive and active systems, where passive systems do not require an external power source and active systems use electricity for pumping or sensing capabilities. Complex solar collectors improve system functionality and generally have low thermal losses at high temperatures.

Passive Systems[edit]

Passive solar water heating systems are appealing due to limited support requirements which reduce cost as well as maintenance requirements. The major limitation of is that they can typically only be installed in areas where there is no risk of freezing. Currently there are many regions where the likelihood of pipe freezing is unknown, thus passive systems cannot be installed. They are generally less efficient than active systems due to their lack of sensing and pumping capabilities, and may not meet the heat demand of a residence if solar resource is insufficient.

Integral Storage Collector Systems (ICS)[edit]

Figure 3. Cross-sectional View of an Integrated Storage System [28]

Integral storage collector systems are the most basic configuration of a passive heating system, and function by combining the hot water storage tank with the collector. These systems generally cost less than other passive water heating systems due to increased simplicity and limited component requirements.

The basic ICS system consists of a hot water tank with an insulated back side, where reflectors inside the tank serve as absorbers and reflect light to the stored water. The tank is covered with a transparent insulating cover to minimize evaporation and prevent water contamination. Figure 3 shows a side view of an ICS system.

The use of a transparent cover for an integral storage collector system introduces the potential for large heat losses. Recent technologies in transparent insulation materials have introduced low-transmittance solutions to minimize losses at the expense of cost increases. The use of a vacuum inside the tank can minimize convective losses, again at the expense of system complexity and cost.

The opportunity for freeze protection is limited for ICS systems due to the basic configuration of these systems. Basic freeze protection can be provided by rooftop mounting, double or triple glazing on the transparent covering, freeze tolerant tank construction or the implementation of an auxiliary heat source. Generally speaking, however, the use of integral storage collector systems is limited to mild climates unless being used as a pre-heat for conventional hot water systems.

Thermosyphon Systems[edit]

Figure 4. Schematic of a Thermosyphon System [28]

Thermosyphon water heating systems function by exploiting water density changes to circulate heated water from a collector to a thermal storage unit, as shown in Figure 4. The collector is installed below the storage tank, which draws water from the bottom of the tank to the collector where it is heated. The heated water then rises back to the storage tank due to its lower density. This cycle continues until the water temperature is constant throughout the tank.

The thermal reservoir for a thermosyphon must be separated from the collector and located a minimum of 18” above the collector for adequate circulation [21]. In general, best practice entails a larger height difference between collector and storage tank to ensure the system doesn’t run backwards at night. This is often achieved by installing the storage tank on the roof, which brings about further structural design considerations.

The major disadvantage of a thermosyphon system is its lack of reliable freeze protection. These systems are inert and cannot react quickly to changes in solar radiation. Collector efficiency generally decreases with high temperatures in the solar cycle, thus these systems may not meet hot water demands for a particular application.

One way to increase the versatility of a thermosyphon heating system is to install a solenoid valve, syphon breaker or air vent to completely drain the collector in the event of freezing weather. These additions increase the complexity of the system and require some, however they offer some degree of freeze protection in regions where a passive system may be otherwise impossible.

Another method for freeze protection in thermosyphon systems is through the use of a thermal fluid (water with antifreeze, for example) in conjunction with a heat exchanger. In colder regions, the thermal fluid may circulate through a tank jacket to provide additional freeze protection. For these systems, piping exposed to the environment must be protected from freezing by use of electric heat tape or automatic drip valves.

The basic thermosyphon system is most applicable for mild climates where freezing temperatures are unlikely. For this reason, these systems are particularly popular in Australia, Japan and Israel. Until a more reliable form of freeze protection is introduced, the widespread use of thermosyphon domestic water heating systems is unlikely in temperate and cold regions.

Phase Change Systems[edit]

Figure 5. Schematic of a Phase Change System (with HEX) [21]

Phase change solar water heating systems operate in a similar fashion to thermosyphon systems, however the thermal fluid is allowed to change phases inside the system. The thermal fluid is heated up in the collector and allowed to vaporize. It then travels upwards to the thermal storage, where it condenses and transferred stored heat to the domestic water supply. The thermal fluid then flows back into the collector by gravity. This process can be observed in Figure 5.

Generally, it is useful to install temperature sensors in phase change heating systems to ensure refrigerant vapor temperature is higher than hot water temperature in the tank. When hot water temperature exceeds that of the thermal fluid vapor, the cycle must be stopped to prevent the system from running backwards.

As with thermosyphon systems, the thermal storage tank in a phase change heating system must be located above the collector to ensure proper functionality. Increased temperature accuracy requirements of a phase change system generally require some sensing capabilities, and the use of a pump can further increase functionality.

Active Systems[edit]

Active solar water heating systems are more common than passive systems due to their increased installation and application versatility. They offer improved circulation, increased sensing and controlling capabilities, as well as improved and more reliable methods of freeze protection.

Forced Circulation Systems[edit]

Forced circulation systems use a pump to move heated water through the system, eliminating the need for a height difference between the collector and the thermal reservoir. The use of a pump allows flow rate adaptation to available solar radiation, and thus can ensure adequate water heating. In general, lower flow rates compromise pump efficiency and high flow rates have higher power requirements; flow rate should be monitored to balance these considerations. Temperature sensing can be employed to monitor temperatures in the collector and storage tank, and can dictate whether or not the pump should run based on temperature difference.

Direct Heating Systems[edit]

Figure 6. Schematic of a Direct Heating System [21]

Direct solar water heating refers to direct heating of the domestic water source by the sun. In general, these systems offer limited methods of freeze protection.

A single tank direct heating system consists of a single tank where potable water is circulated through the collector to a single storage tank. An auxiliary heat source is generally installed at the top of the tank to ensure tank stratification for proper system functionality. Cold water is supplied to the collector from the bottom of the tank, and the hot water leaving the collector enters the tank at the top. Sensing and controller capabilities allow controlled circulation when tank stratification is insufficient for adequate water heating. This configuration can be found in Figure 6.

Freeze protection for direct heating systems is limited to recirculation and draindown systems. Recirculation can be used in regions where freezing temperatures are less of a concern. These systems function by continually circulating water from the storage tank through the collector to prevent system freezing. Temperature sensors and controls can prompt recirculation in cooler weather.

Draindown systems contain motorized or solenoid valves which are used to drain the system when freezing is a possibility. In freezing conditions, the water supply to the collector is cut off and a drain valve is opened; air is allowed to enter the collector and associating piping to avoid damage due to freezing.

Indirect Heating Systems[edit]

Figure 7. Schematic of an Indirect Heating System [21]

Indirect water heating methods were developed to provide a more reliable method of freeze protection than what is offered by direct solar water heating systems. The general design consists of a non-freezing thermal fluid which is heated in the collector, and heat is transferred from the thermal fluid to the domestic hot water source in a heat exchanger as found in Figure 7. These systems may contain recirculation or draindown systems as described above to further protect against freezing.

Important considerations for indirect water heating systems include increased pumping requirements, heat transfer characteristics, capital costs and ongoing maintenance requirements. The expansion of the heat transfer fluid must be taken into account when designing an indirect heating system, as well as location and installation of check valves. It is imperative that the thermal transfer fluid is in a closed loop, as the pressure of city water supply will otherwise prevent draining in the event of a freeze.

It is possible to use air as the thermal fluid, however efficiency can be limited due to its lower heat capacity. Additional considerations such as the cost of blower power, heat exchanger efficiency and system air tightness must be compared to the costs of using a traditional liquid thermal transfer fluid.

Design Considerations[edit]

Solar water heating systems must be designed primarily as a function of available solar irradiation and hot water demand. Ultimately, the implementation of a solar water heating system depends on its ability to save the user money, protect the environment and limit use of fossil fuels. Once these design parameters are well understood, an appropriate solar water heating system may be selected.

Proper design selection is essential to ensure maximum gain to the user. Solar domestic water heating systems are only cost-effective when properly designed, as performance depends directly on installation configuration. The selected design can be validated by knowing available solar insolation, heat and load demands and solar fraction. Correct design installation is essential to proper functionality of any system.

Table 1 below outlines some basic considerations which should be taken into account when selecting a solar water heating system for a particular application.

Table 1. Design Considerations [21]

Other operating considerations include freeze protection, a means for rejecting excess heat to prevent system boilover, the potential need for auxiliary heat back-up if the solar system fails and contamination prevention for indirect heating with a non-potable thermal fluid. These considerations are explored in more detail below.

Solar Resource[edit]

Other operating considerations include freeze protection, a means for rejecting excess heat to prevent system boilover, the potential need for auxiliary heat back-up if the solar system fails and contamination prevention for indirect heating with a non-potable thermal fluid. These considerations are explored in more detail below.

Heat and Load Demand[edit]

Figure 8. Estimated Residential Hot Water Load Profile [21]

Hot water draw can vary greatly from site to site and season to season, and also based on the application (residential vs. commercial, steam vs. hot water). While hot water use can be estimated based on usage at other locations, the only way to properly quantify hot water demand for a specific site is by measuring use over time. It is generally accepted that the highest hot water usage is between 6am and 10am, with a second smaller peak from 2pm to 10pm, as found in Figure 8.

As a point of reference, Table 2 below outlines the hot water demand of a residential building in Germany in 1982.

Table 2. Hot Water Demand of Residential Buildings in Germany - 1981 [21]

While these values can be used as a template for system design, it is important to note that unexpected load variations from day to day can significantly affect system performance. Other demand considerations include heat losses, heat-up considerations and solar collector power output effect on auxiliary heat needs.

Once the hot water demands of a particular location are well-defined, the associated heat demand can be calculated as follows:

KelseyMTUEquation10.jpg

This value can be used to determine the solar capabilities of a particular region with respect to demand for a specific application.

Hot Water Demand in the United States[edit]

It is estimated that a typical American family of 4 uses 50-100 gallons of hot water per day, equating to approximately 50,000 Btu/day [21]. In sunny regions, one square foot of collector area can typically harness 500 Btu/day, making a solar water heating system applicable for these regions and minimizing the need for auxiliary energy.

A solar water heating system was installed in Salt Lake City, Utah for a residence of 12 young girls and their children [4]. Measured hot water demand was as follows:

Table 3. Measured Hot Water Use for Young Adults in Utah [4]

Again, these values can be used as guidelines, however accurate hot water demand for a specific location can only be verified by direct measurement.

Hot Water Demand in South Africa [25][edit]

One appealing aspect of solar water heating systems is their potential application in developing regions. It has been suggested that Americans use up to seven times the amount of hot water that is used in other developed European countries, thus hot water use cannot be extrapolated to estimate use in developing area. Cost is of particular importance in these environments, as installation will only be considered if significant cost savings are achievable.

A study was performed by Joshua Meyer for the ASHRAE (American Society of Heating, Refrigerating, and Air-Conditioning Engineers) on hot water demand in South Africa as a function of living space. South Africa has a mild climate, thus water heating in middle to upper-class homes is the largest source of energy consumption in the domestic sector, accounting for 40-50% of total energy costs for a household.

Current methods for water heating in South Africa include electric element heating, gas heating, heat pumps and some solar heating, where electricity dominates as the main method for domestic water heating. There are a limited number of homes equipped with solar water heating systems due to their high capital cost and the low cost of electricity.

In 1996, hot water consumption over a year was measured by the Johannesburg Metropolitan Area for 770 homes, ranging from affluent residences with readily available hot water to shacks outside the city center where water is heated over an open fire. It was found that daily hot water consumption was higher in higher income homes, and that hot water consumption increases approximately 70% in winter months, especially by upper-class (low occupant density) homes. In general, hot water use spikes occur from 7am to 9am, and again from 9pm to 11pm.

Daily consumption was measured based on residential occupant density, where lower-income homes generally have higher occupant density. Table 4 below lists estimated daily residential consumption rates by occupant density.

Table 4. Hot Water Consumption for Winter Months in Johannesburg, South Africa [25]

While the values observed in this study cannot be applied directly to the design of a solar water heating system in all developing areas, they are qualitatively useful at the beginning of the sizing and design process.

Solar Fraction and Cycle Efficiency[edit]

Solar fraction refers to the percentage of overall heat demand that is provided by the solar system, defined mathematically as:

KelseyMTUEquation11.jpg

A solar fraction of 50-60% is desirable for domestic water heating applications, as this allows a compromise between heating capability and cost considerations [28]. In regions of high solar irradiation, the solar fraction may be much higher, especially in areas where there is minimal temperature variation from season to season.

After construction considerations, solar fraction is the dominant factor on solar cycle efficiency, where overall system efficiency increases with increasing solar fraction. Solar cycle efficiency refers to total efficiency for the solar thermal system, defined as mathematically as:

KelseyMTUEquation12.jpg

Temperate regions generally have a solar cycle efficiency of 20-50% due to a lower available solar fraction [28].

Design Validation Techniques[edit]

Validation of a selected solar water heating system can be performed to ensure adequate system performance for a specific application. There are two basic methods for optimization: correlation-based methods which base functionality on utilizability, and simulation-based methods using computer programs. Correlation-based methods are limited because of the complexity of solar water heating systems; most methods only allow identification of a single system design by optimizing one function. Computer programs offer a more comprehensive approach for design validation, however their use may increase cost of a particular system.

While there is no widely-accepted method for design validation, a few techniques have been proposed in industry and in the scholarly arena.

Model for Determination of Design Space [19][edit]

One technique proposed by Kulkarni, Kedare and Bandyopadhyaya incorporates design constraints to compare solar water heating designs based on balancing collector area and storage volume. It is found that there is a minimum and maximum possible storage volume for a given solar fraction and collector area. Similarly there exists a minimum and maximum possible collector area for a given solar fraction and storage volume. The overall system is then optimized by minimizing annual costs.

Minimum collector area is determined by finding the smallest possible storage volume to still meet domestic water heating demands. A collector with smaller area than this specified minimum will not meet the thermal requirements of the water heating system.

Larger tank volumes have fewer temperature fluctuations, thus lower temperature water is sent to the collector which increases cycle efficiency. Conversely, increased surface area of these units increases thermal losses which decreases thermal storage efficiency. As storage volume increases, increases in collector efficiency before less noteworthy and changes in storage efficiency become more considerable. Minimum storage volume is defined by the smallest acceptable storage volume to prevent system boiling.

These qualitative considerations can be combined to determine acceptable design limits. Stored water must meet load requirements without boiling; any design which fits between these temperature constraints represents a design which satisfies load requirements and temperature constraints.

While the basic methodology proposed in this paper considers only temperature and load constraints, the model can be extended to include cost considerations, solar fraction and year-to-year performance data, which can be useful for economic optimization of a specific design. Incorporation of additional constraints will increase the number of required iterations, however the result is a collection of acceptable designs for one particular application.

Results using this methodology cannot be extrapolated globally due to uncertainty in solar insolation data, system parameters and cost data, however it still provides a useful way to determine a collection of acceptable designs for a particular domestic water heating application. This technique is especially useful in retrofit applications by incorporating existing constraints into the global model. Uncertainties associated with system parameters may cause variance between a globally optimized system and actual field results, thus results are more effectively applied qualitatively.

Procedure for Life Cycle Assessment [3][edit]

The completion of a life cycle assessment is useful to understand the economics of solar water heating and as to identify major contributors to environmental impacts and compare design options for a particular application. A complete life cycle assessment requires in depth planning, inventory analyses, impact assessments and improvement analyses, however a more generalized approach is used in “Life Cycle Assessment of Built-in-storage Solar Water heaters in Pakistan” (Asif and Muneer).

The purpose of this study was to determine the effect of including fins on a collector for a conventional integrated collector storage system. The comparison process was simplified by installing two identical systems were constructed: one using the traditional collector design and one with fins. Both systems were fully instrumented to measure hourly variation of ambient air and water temperatures in addition to tank stratification temperatures. Data was collected for four summer months and output power for each system was compared.

Table 5 below outlines system characteristics of each design.

Table 5. Integral Storage Collector System Comparison by Materials and Cost [3]

The parameters above describe the more common measurement tools used to perform a life cycle assessment of a particular system. It is also beneficial to consider the energy demand and savings achieved by installing a solar water heating system.

Based on the materials used for the solar water heaters (stainless steel, glass, rubber, timber and glass wool insulation), energy values per mass were determined as found in Table 6 below. This information combined with average amounts of transmitted energy can be used to estimate the energy payback period for both systems.

Table 6. Integral Storage Collector System Comparison by Energy Consumption [3]

The methodology used to determine energy demand and savings for these two systems can be applied to examine carbon emission savings. It is estimated that the use of solar water heating systems reduces carbon emissions on the scale of 0.02kg/MJ. This estimation is extrapolated for both designs as found in Table 7 below.

Table 7. Integral Storage Collector System Comparison by Carbon Footprint [3]

The results found in this paper cannot be applied to all solar water heating systems, however the methodology can be extended to compare an iterated system to a more basic or traditional system.

Installation Considerations[edit]

Correct installation of a solar water heating system is required for proper system functionality. Table 8 below describes some specifics important points for system installation.

Table 8. System Implementation Considerations [21]

These considerations can be further divided as found below.

Freezing[edit]

Figure 9. Contour Map Describing Likelihood of Pipe Freeze by Region [29]

One important design consideration is the potential for freezing in temperate or cold climates. Exposed piping is at the greatest risk for freezing and permanent damage can be caused to the collector during freezing weather. For this reason, the installation of passive systems is limited to regions where freezing weather is not a concern.

Daytime water draw has a large effect on the likelihood of system freezing and thus must be considered when designing the freeze-proofing characteristics of a system. Increased hot water draw can replace cold water on the verge of freezing with warmer water, which also may melt existing ice buildup. Other factors which may inhibit pipe freeze include conduction, single-pipe convection and overall solar gain.

A variety of studies have been performed to investigate the mechanism and likelihood of pipe freezing. A University of Illinois study by Gordon in 1996 revealed that pipe burst due to freezing is only caused when two complete ice blockages occur. Another study at the University of Wisconsin by Beckman and Bradley in 2000 created a time-to-freeze map of the United States for a wide variety of pipe configurations. Salasovich, Burch and Barker of the National Renewable Energy Laboratory used the information from these studies to create a more descriptive contour map which details the likelihood of pipe freezing based on real weather data, hot water draw information, vacation periods with no hot water draw and pipe geometry and insulation, found in Figure 9.

While the figure above provides useful estimations of where pipe freezing is a concern, it can only be used qualitatively. Additionally, the figure above is only applicable for metal pipes; polymer pipes have higher yield strain and will not necessarily burst due to complete blockage.

Sensor Installation[edit]

Sensors and controls can be installed on a solar water heating system to more accurately monitor and adjust heating characteristics based on feedback. It is especially important to ensure proper sensor selection and installation, well as to determine optimum sensor location. Best practice entails that the estimated temperature difference should not exceed the precision range of the sensor, and the device should not be in direct contact with the fluid being measured.

Collector Array Performance[edit]

When possible, it is preferable to have solar collectors face due south and have a tilt angle approximately equal to the installation latitude [21]. Additional architectural considerations may be required in locations which prevent these installation guidelines, such as tall buildings or areas with excessive shade.

Improper piping of collector arrays can cause flow imbalance which will eventually overheat sections of the collector and decrease overall collector efficiency. This can be avoided by limiting total piping length via series installation of the collectors. This may solve some efficiency issues, however collectors should not be installed with more than three in series due to increased pressure requirements and excessively high inlet temperatures for the final collectors.

A final design consideration for large collector arrays is the increased difficulty to monitor and detect circulation issues. More recent infrared thermographic technologies can be used to show temperature distribution throughout the system which can indicate flow imbalances, air blockages or broken collectors.

Additional Considerations[edit]

Cost[edit]

Cost reduction of existing solar water heating systems is essential to achieve a substantial market in the United States. Current system prices are not low enough to gain widespread popularity unless fossil fuel prices increase radically.

The cost of solar heated water systems is a function of system efficiency, solar climate, capital investment and operating costs. Cost considerations can include design and construction costs, life cycle cost or the cost of energy delivered by the system. Many of these parameters are site-specific and thus cannot be easily compared from system to system. Further difficulties arise when trying to quantify highly variable costs such as maintenance and operating costs or variable gains such as tax credits or other tax effects.

A life cycle analysis is the most comprehensive method to determine the most cost-effective system for a particular application. Reliable cost information is difficult to obtain, particularly for residential systems and larger-scale solar thermal systems. Ultimately it is essential that designers can assess cost effects of various design choices prior to design selection.

Cost Estimates by Component [21][edit]

One way to compare costs from system to system is by estimating component costs; total system cost is then a matter of considering which components were used.

In the textbook “Active Solar Systems,” George Lof describes component costs for solar water heating systems. Data on residential component costs is unavailable, however the information for commercial systems can be qualitatively applied to residential systems; system costs will typically be slightly lower than for residential systems due to the concept of “economy of scale.” There also may be some regional variation of labor and materials costs.

In general, collector array costs are independent of application and more contingent on collector type. This report found the following information in 1981 for collector array costs.

Table 9. Cost Estimates by Collector for Commercial Systems [21]

Support structure costs are dependent on site characteristics, collector selection and other functional considerations. In systems where collector rests directly on the existing roof or does not require additional structural elements, support structure costs can be minimal. Little cost variation was observed between roof-mounted and ground-mounted structures; general support structure costs are organized according to collector type, found in Table 10 below.

Table 10. Cost Estimates for Storage by Collector for Commercial Systems [21]

Storage tank costs are expressed in terms of storage capacity and depend on type and location of vessel. Generally, unpressurized tanks are the most inexpensive, followed by fiberglass, rock bins and finally pressurized steel tanks. Buried storage is the lowest cost storage location, followed by external tanks, then internal tanks. Specific cost data for these thermal storage options was unavailable.

Other cost considerations include energy transport costs and sensing and controller costs. Generally, energy transport costs for hot water systems are lower than for steam applications. Controls costs are highly dependent on application, but are generally higher for steam applications than hot water applications. Additional construction costs are site-specific and do not show any identifiable trends.

Cost Estimates by Breakeven Cost [10][edit]

Figure 10. Residential Solar Water Heating System Breakeven Cost [10]

Another method of comparing solar water heating systems is to determine system breakeven cost, which is defined as the point when the cost of energy saved with a solar water heating system equals the cost of conventional water heating. This method of system comparison takes into account all financing and incentives for installation of a solar heating system and discounts benefits of reduced electric and natural gas bills.

One analysis, performed by the National Renewable Energy Laboratory, examines breakeven cost for solar water heating systems in the United States compared to conventional natural gas and electric systems. It was found that the main criteria affecting breakeven cost are fuel price, local incentives and other technical factors such as solar resource, system size and hot water demand; local incentives largely drive the cost-efficiency of solar water heating systems. The highest breakeven costs occur in the southwest, which has a good solar resource, and the northeast where electricity is expensive.

Findings from this study were published in the form of a map where breakeven solar heating system costs is given by location as found in Figure 10.

More generally, breakeven costs can be grouped according to the percentage of homes which could benefit financially from a solar water heating system at a particular price. These results are outlined in Table 11 below.

Table 11. Estimated Solar Water Heating System Breakeven Costs [10]

As the price of solar water heating systems is reduced, they will become more cost-effective for a wider range of homes. This cost reduction is necessary for widespread application of solar water heating technologies.

Cost Reduction Methods for Cold Climate Systems [8][edit]

A study performed by Salasovich, Burch and Hilman considers cost reduction methods for solar water heating systems with freeze protection. It was determined that overall system cost can be reduced through the use of lower-cost components, replacement of metal components with polymeric materials and overall system simplification.

The largest cost reduction found was by replacing conventional pressurized solar tanks with unpressurized polymer tanks, achieving cost reductions of up to 17%. The replacement of existing heat exchangers with immersed polymer heat exchangers can achieve cost reductions of up to 9%. While the use of polymeric material can adversely affect system performance, the associated decrease in cost is proportionally more significant.

System Installation and Maintenance[edit]

Installation, monitoring and servicing of a solar water heating system are as important as proper system design selection. Improper or inadequate attention to these steps contributes poor system performance, accelerated deterioration and eventual system destruction. Published handbooks outline installation guidelines as well as monitoring procedures for a variety of solar water heating systems. A summary of these guidelines are outlined below.

Installation instructions should include step-by-step installation and removal instructions for each system component, as well as relationships between components and interfacing techniques with the physical location of the system. Before the system is put into regular use, system operation and controls should be verified and all piping, valves and connections should be certified leak-free. Instructions should also outline normal and emergency start-up and shut-down procedures, with particular emphasis on critical temperature, pressure and flow values. Any controls, sensors, dampers or valves should be labeled according to function and flow direction to ensure simplified maintenance in the event of an emergency.

A manual detailing routine maintenance should be included in the system, with specific procedures for required maintenance such as periodic component servicing, fluid testing and system flushing. This document should also include information on hazards that may be encountered while servicing the system. Finally, monitoring instructions should be included which detail required procedures to properly monitor the system and instructions for use of simple sensors and indicators which reveal proper system operation.

There are two standardized monitoring methods which have been used in the National Demonstration Program, a government initiative to raise awareness about solar technologies. The first method is known as the National Solar Data Network (NSDN), which analyzes solar water heating systems based on the parameters outlined in Table 12 below.

Table 12. Description of National Solar Data Network Monitoring Criteria [21]

Proper determination of the parameters above allow for simple and accurate system-to-system comparisons.

A second monitoring technique was developed by the Northeast States Solar Water Initiative for systems installed as part of this project. Monitoring objectives are outlined in Table 13 below.

Table 13. Description of Northeast States Solar Water Initiative Monitoring Criteria [21]

This monitoring program is less specific than the NSDN system, however the required information can be collected more easily through simple instrumentation and manual data logging. The most pertinent limitation of this program is its inability to determine heat loss by component. Again, results can still be applied qualitatively.

Residential System Monitoring[edit]

While the monitoring programs outlined above are useful for large-scale solar water heating system monitoring, there are more simple techniques which can be employed at home to ensure proper functionality of a solar water heating system.

First, it is important that the owner is educated on proper functionality, maintenance and servicing of their system. The user should be capable of troubleshooting routine issues such as collector cleaning and snow removal, and of recognizing signs of more critical issues such as system boilover or back-siphoning which can cause system freezing. One additional concern is that new home owners may lack expertise or interest in a system previously installed in their home.

Residential systems should include accurate and easy-to-ready instrumentation for temperature, pressure, flow and component-specific energy use. Sensors, controllers and valves should be periodically inspected to ensure proper functionality and avoid issues due to improper sensing. In indirect heating systems, there may be an occasional need to add glycol due to leakage or high levels of acidity. Also, solar-heated water can be much hotter than conventional domestic hot water. One method to remedy this issue is through the use of a tarp or sunscreen to cover the solar collector when delivery temperature exceeds what is comfortable for the user. Finally, the system should be prepared for periods of non-use (such as vacations) to avoid system overheating.

Other issues widely encountered with residential solar water heating systems are outlined in Table 14 below, along with why these issues are a concern. The repeated instances of these issues have led US Department of Housing and Urban Development to provide specified residential solar water heating system requirements for all residential solar water heating systems.

Table 14. Revised HUD Solar Intermediate Minimum Property Standards [21]

Through the guidelines outlined above, proper maintenance and monitoring of a residential solar water heating system can be more easily completed. As more installation experience is obtained, fewer issues will be encountered during implementation of solar water heating systems.

Commercial System Monitoring[edit]

In general, commercial solar water heating systems have similar issues to those encountered with residential systems. Because commercial systems typically require more complex controllers than residential systems, additional areas of concern include controls, sensors, freezing and collector or collector array distribution. While many of these issues can be resolved more quickly than in the past, obstacles remain with probe accuracy, tracking accuracy, optimum differential offset and hysteresis.

Demonstrations and Dissemination[edit]

Solar water heating system design development has been largely completed by the solar industry, as government support has traditionally focused on demonstration and performance evaluation techniques. This is in contrast to the majority of other solar technologies which have evolved mostly due to government funded research and development and tax subsidies for “green energy” users. Federal funding designated towards solar water heating system design has been limited to materials development, component development, controls and other non-engineering aspects.

One program, the National Solar Water Heater Workshop, was developed in Tempe, Arizona in 1974. The 10 year project was started by Stanley Mumma with federal funding with the intention of establishing staff, working through technical aspects of solar water heating, creating workshop execution plans and preparing documents for the project. The purpose of the project was to project the importance and relevance of solar water heating technologies to universities and the public via the development of a system that could be installed in residences nation-wide.

Currently, the NSWHW program is no longer running and many of their installed systems are no longer in operation. Despite the termination of the program, it has illuminated some important realities of the solar water heating industry. Solar water heating system components should be function for 30 or more years; damage to components is generally the result of operator error. It is speculated that system longevity is limited by lack of market and service sector rather than usefulness and cost-effectiveness of current systems.

Currently, governments use three basic instruments to support renewable energy technologies. The first is tax and market-based incentives, which have been widely implemented in the United States and largely drive cost-effectiveness of current systems. Environmental regulations have also been implemented to reduce the use of fossil fuels in the commercial sector. Finally, direct expenditures have been directed towards the development of renewable technologies, however this is less applicable to the solar water heating industry. For a substantial market to exist, these tools must be applied to positive externalities of technology development, environmental benefits, R&D and demonstration projects, and general program preparation, education and training. [2]

Other groups promoting solar technologies are industry organizations and non-governmental organizations (NGO’s) associated with energy efficiency or environmental concerns. For the future, it will be increasingly important to consider new markets for renewable energy technologies, requiring increased decision-making and participation from a variety of stakeholders.

To the Future[edit]

Future Expectations[edit]

Some people expect marketing and distribution costs of solar water heating systems to go down, however current manufacturers do not consider such changes to be likely. Because a sacrifice in product quality is a deterrent to the purchase of solar water heaters, increased prices continue to be a reality. While there are fewer marketing and distribution costs, solar water heaters are simply not competitive without subsidies, tax benefits and other incentives. Despite these realities, it is expected that commercial manufacture and sale of solar water heaters, primarily in the residential sector, will continue at current levels. One hope is that solar energy construction costs will be reduced as the benefits of experience in design and installation of solar components are available. Improvements in product quality will lead to greater system reliability, appeal and user confidence; reducing marketing costs and increasing overall demand.

In general, it is estimated that as energy industry regulations increase, further competition will validate previous investments in renewable energy technologies. This, combined with the potential for cost reduction, will greatly expand the market beyond where it is today. As more developing countries introduce their own environmental policies on fossil fuels, the environmental advantages of solar technologies will be more widely applicable. Ultimately, however, more workable and focused policies are required to drive development of these systems, both to increase their appeal as well as to reduce future uncertainties associated with fossil fuel consumption [2].

Applications in Developing Communities [23][edit]

One exciting application for solar water heating systems is their use in developing regions where a consistent supply of hot water is otherwise unavailable. As technologies and public policy mature, these systems will be increasingly realistic for sustainable development applications. Donor programs are taking more market-based approaches to respond to local demands, increasing the likelihood of long-term success. Ultimately, however, success is contingent on new business models, rural entrepreneurship and other subsidy advantages coupled with increased emphasis on social benefits.

Hot water for residential and commercial uses can be cost-effectively provided to developing regions using solar water heating technologies. Currently 2/3 of all systems are installed in developing countries, especially in China, Egypt, India and Turkey. Many of these markets are driven by government requirements, however in general solar water heating systems can be effectively implemented in areas which are already undergoing development as energy technology is integrated with other changes.

The most important factor for system implementation in developing regions is the required behavior changes required by the user after implementation. Many development agencies have promoted small-scale renewable energy technologies, but most projects fail due to poor technical performance and limited sustainability based on user needs and local conditions. Issues also arise with cost, availability of system components and system maintenance.

It is difficult to quantify the impact of renewable energy technology implementation in developing areas. It is clear that implementation of solar water heating systems in these areas provide significant social benefits and an improved quality of life, however economic benefits are less clear. As experience with productive uses of renewable energy is increased, sustainable implementation of these systems can be improved.

Re-design Considerations[edit]

Because of the maturity of solar water heating technology, it is difficult to suggest “quick” changes to systems to improve system functionality. Instead it is more pertinent to be aware of available technologies and compatibility based on specific application. While solar water heating offers a reliable, sustainable method to reduce fossil fuel use, success ultimately depends on proper system selection, installation and maintenance. As design evaluation and validation methods improve, this technology can have a wider and more practical positive impact on our global society.

Conclusion[edit]

Reliable and sustainable energy sources are required because current consumption of energy cannot be sustained. The current use of energy is responsible for greenhouse effect, acid rain and other negative impacts on public health and the environment. The impacts of renewable energy can reduce these effects and help to slow the inevitable consumption of earth’s remaining fossil fuels.

Solar water heating provides a feasible alternative energy technology to replace fossil fuel-power conventional water heating methods. There are an estimated 10EJ of global energy potential for solar water heating applications, however current use is limited to approximately 2.2% of this potential [27].

Current system limitations such as freeze protection, reliability and cost in particular will continue to hinder the market until significant cost reductions are achieved. Generally, the appeal of solar water heating is currently limited to motivation by incentives besides cost, such as emission reduction or use of “green” technologies.

Works Cited[edit]

[1] Altman, Manfred. Conservation and better utilization of electric power by means of thermal energy storage and solar heating. Philadelphia: University of Pennsylvania, Towne School of Civil and Mechanical Engineering, 1971. Print.

[2] Anderson, D. (1997). Renewable energy technology and policy for development. Annual Review of Environment and Resources, 22(15435938), 187-187. Retrieved from http://search.proquest.com/docview/219889969?accountid=28041

[3] Asif, M., & Muneer, T. (2006). Life cycle assessment of built-in-storage solar water heaters in Pakistan. Building Services Engineering Research & Technology, 27(1), 63-69. doi:10.1191/0143624406bt141tn

[4] Atkinson, G., & Colvin, T. (2009). Solar hot-water heating system. ASHRAE Journal, 51(9), 44-50,53. Retrieved from http://search.proquest.com/docview/220474118?accountid=28041

[5] Baylin, Frank. Low Temperature Thermal Energy Storage: A State-of-the-art Survey. Golden, CO: Dept. of Energy [Office of Energy Technology], Solar Energy Research Institute, 1979. Print.

[6] Beckman W. and Bradley D. (2000) Three-season Operation as a Method of Freeze Protection in Solar Domestic Hot Water Systems. In NREL Subcontract Report for Subcontract ZDC-8-17675-08.

[7] Boyle, Godfrey. Renewable Energy. Oxford: Oxford UP in Association with the Open University, 2004. Print.

[8] Burch, J., J. Salasovich, and T. Hillman. "Cold-Climate Solar Domestic Water Heating Systems: Life-Cycle Analyses and Opportunities for Cost Reduction." National Renewable Energy Laboratory (2005). Print.

[9] Burkhardt, J., G. Heath, and C. Turchi. 2011. Life Cycle Assessment of a Parabolic Trough Concentrating Solar Power Plant and the Impacts of Key Design Alternatives. Environmental Science & Technology. 45: 2457–2464. http://dx.doi.org/10.1021/es1033266

[10] Cassard, Hannah, Paul Denholm, and Sean Ong. "Break-even Cost for Residential Solar Water Heating in the United States: Key Drivers and Sensitivities." National Renewable Energy Laboratory (2011). Print.

[11] Duffie, John A., and William A. Beckman. Solar Engineering of Thermal Processes. Hoboken, NJ: Wiley, 2006. Print.

[12] Gamage, J., & Zhang, Z. (2010). Applications of photocatalytic disinfection. International Journal of Photoenergy, (1110662), n/a. Retrieved from http://search.proquest.com/docview/855956756?accountid=28041

[13] Gilbert, John. "There Is Greater Potential from Solar Energy at Higher Latitudes." Architects Journal (2006). Print.

[14] Golob, Richard, and Eric Brus. The Almanac of Renewable Energy. New York: H. Holt, 1993. Print.

[15] Gordon, Jeffrey R. "An Investigation Into Freezing and Bursting Water Pipes in Residential Construction." GreenBuildingAdvisor.com. University of Illinois Urbana-Champaign, 1996. Web. 23 Apr. 2012. <http://www.greenbuildingadvisor.com/blogs/dept/green-building-blog/jeffrey-gordons-paper-bursting-pipes>.

[16] Harvey, Leslie Daryl Danny. A Handbook on Low-energy Buildings and District-energy Systems: Fundamentals, Techniques and Examples. London: Earthscan, 2006. Print.

[17] Juanicó, Luis. "A New Design of Roof-integrated Water Solar Collector for Domestic Heating and Cooling." Solar Energy 82.6 (2008): 481-92. Print.

[18] Klapp, Jaime, Jorge L. Cervantes-Cota, and José Federico. Chávez-Alcalá. Towards a Cleaner Planet: Energy for the Future. Berlin: Springer, 2007. Print.

[19] Kulkarni, G., S. Kedare, and S. Bandyopadhyay. "Determination of Design Space and Optimization of Solar Water Heating Systems." Solar Energy 81.8 (2007): 958-68. Print.

[20] Kumar, Atam. "Novel Design Concepts in Solar Water Heating." Solar Water Heating Systems: Proceedings of the Workshop on Solar Water Heating Systems (1986): 337-45. Print.

[21] Löf, George. Active Solar Systems. Cambridge, Mass. [u.a.: MIT, 1993. Print.

[22] Lunde, Peter J. Solar Thermal Engineering: Space Heating and Hot Water Systems. New York: Wiley, 1980. Print.

[23] Martinot, E., Lew, D., Moreira, J. R., & Wamukonya, N. (2002). Renewable energy markets in developing countries. Annual Review of Environment and Resources, 27(15435938), 309-309. Retrieved from http://search.proquest.com/docview/219881326?accountid=28041

[24] Metal roofs as solar collectors. (2005). Energy Design Update, 25(2), 1-3. Retrieved from http://search.proquest.com/docview/219443716?accountid=28041

[25] Meyer, J. P. (1999). Domestic hot water consumption of the developed and developing communities in South Africa. ASHRAE Transactions, 105(00012505), 173-173. Retrieved from http://search.proquest.com/docview/192553904?accountid=28041

[26] Mumma, Stanley A. "Over Thirty Years of Experience with Solar Thermal Water Heating." American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. 117.1 (2011): 57-63. Print.

[27] Pillai, Indu R., and Rangan Banerjee. "Methodology for Estimation of Potential for Solar Water Heating in a Target Area." Solar Energy 81.2 (2007): 162-72. Print.

[28] Quaschning, Volker. Understanding Renewable Energy Systems. 3rd ed. London, UK: Earthscan, 2005. Print.

[29] Salasovich, Jim, Jay Burch, and Greg Barker. "Geographic Constraints on Passive Solar Domestic Hot Water System Due to Pipe Freezing." Solar Energy 73.6 (2003): 469-74. Print.

[30] Sen, Z., Sahin, A.D., 2001. Spatial interpolation and estimation of solar irradiation by cumulative semivariograms. Solar Energy 71 (11), 11– 21.

[31] Sioshansi, Ramteen & Denholm, Paul. The Value of Concentrating Solar Power and Thermal Energy Storage. National Renewable Energy Laboratory, Feb 2010. Web. 26 Feb. 2011. <http://www.osti.gov/bridge>.

[32] Smyth, M., and H. Skates. "A Passive Solar Water Heating System for Vineyard Frost Protection." Solar Energy 83.3 (2009): 400-08. Print.

[33] Smyth, M., P.C. Eames, and B. Norton. "A Comparative Performance Rating for an Integrated Solar Collector/storage Vessel with Inner Sleeves to Increase Heat Retention." Solar Energy 66.4 (1999): 291-303. Print.

[34] Tsilingiris, P. T. "Design and Performance of Large Low-cost Solar Water Heating Systems." Renewable Energy 9.1-4 (1996): 617-21. Print.