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Solar hot water

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Solar hot water describes active and passive solar technologies that utilize the freely abundant solar thermal energy in order to heat water for a desired application.

It is one of the most efficient ways to heat water (in terms of energy/waste), as it requires no energy conversion, unlike electric-resistance heating or fuel burning. It is a simple transfer and concentration of heat energy from one place to another. (See Wikipedia:Heat transfer.) Another example of this technology's efficiency is it runs on solar energy, which is free, and only dependent on the technology used, and its cost and efficiency. In other words, the energy is free, only the collection, conversion, and storage devices contribute to the cost of the system.

If you have ever felt hot water trickle out of a garden hose that has been sitting in the sun, you’ve experienced solar hot water in action.

Essentially, a solar hot water system is made up of a solar thermal collector, a well-insulated storage container, and a system for transferring the heat from the collector to the container vis-à-vis a fluid medium, which in some cases is the water itself.


Being as there are countless applications using domestic, commercial, and industrial hot water globally, there are opportunities to apply solar thermal technologies to heat this water.

Today the market is changing and both the economic and environmental costs associated with using gas and electricity to heat water are being challenged by more efficient, less costly systems like the solar hot water system.


Solar hot water is not a new phenomena. It was widely used in the United States up into about the 1920's when it was displaced by reliable fossil fuel systems.

Hot water is considered by some to have little application in the field of appropriate technology and to mostly be a luxury afforded by the developed world. One text[verification needed] on the subject suggests that what hot water is needed in the "3rd World" can be heated using a fuel such as wood that simultaneously heats the home and water. Such dismissals are dangerous on two counts:

  • First, Appropriate Technology aims to reduce waste and increase efficiency in use of natural resources, while wood does heat both water and the home, it is also a natural resource not available in many impoverished countries. Whereas the sun is present everywhere and will be sending out energy regardless of whether we use it. Many women and children in 3rd world countries die of lung disease caused by incorrect ventilation and excess smoke from cooking fires, it actually tends to be the number 1 killer above aids and starvation.
  • Second, It is imperative that where there is a need for hot water, there is a way to get that hot water cost-effectively and within the parameters set by the local resources. This technology, if spread, could cause a significant reduction in the size of a regions ecological footprint associated with conventional means of heating water.

Energy from the Sun[edit]

Map A and B Theoretical annual mean insolation, at the top of Earth's atmosphere (top) and at the surface on a horizontal square meter.
Map C Map of global solar energy resources. The colours show the average available solar energy on the surface (as measured from 1991 to 1993). For comparison, the dark disks represent the land area required to supply the total primary energy demand using PVs with a conversion efficiency of 8%.

Solar radiation reaches the Earth's upper atmosphere at a rate of 1366 watts per square meter (W/m2).[1] Map A shows how the solar energy varies in different latitudes.

While traveling through the atmosphere, 6% of the incoming solar radiation (insolation) is reflected and 16% is absorbed resulting in a peak irradiance at the equator of 1,020 W/m². Average atmospheric conditions (clouds, dust, pollutants) further reduce insolation by 20% through reflection and 3% through absorption. Atmospheric conditions not only reduce the quantity of insolation reaching the Earth's surface but also affect the quality of insolation by diffusing incoming light and altering its spectrum[2].

Map C shows the average global irradiance calculated from satellite data collected from 1991 to 1993. For example, in North America the average insolation at ground level over an entire year (including nights and periods of cloudy weather) lies between 125 and 375 W/m² (3 to 9 kWh/m²/day).[3] This represents the available power, and not the delivered power. At present, photovoltaic panels typically convert about 15% of incident sunlight into electricity; therefore, a solar panel in the contiguous United States on average delivers 19 to 56 W/m² or 0.45 - 1.35 kWh/m²/day.[4]

The dark disks in Map C on the right are an example of the land areas that, if covered with 8% efficient solar panels, would produce slightly more energy in the form of electricity than the total world primary energy supply in 2003.[5]. While average insolation and power offer insight into solar power's potential on a regional scale, locally relevant conditions are of primary importance to the potential of a specific site.

After passing through the Earth's atmosphere, most of the sun's energy is in the form of visible and infrared radiation. Plants use solar energy to create chemical energy through photosynthesis. Humans regularly use this energy burning wood or fossil fuels, or when simply eating the plants, imagine if we found a way to harness this energy leaving plants and fossil fuels out of the equation.

A recent concern is global dimming, an effect of pollution that is allowing less sunlight to reach the Earth's surface. It is intricately linked with pollution particles and global warming, and it is mostly of concern for issues of global climate change, but is also of concern to proponents of solar power because of the existing and potential future decreases in available solar energy. (About 4% less solar energy is available at sea level over the timeframe of 1961–90,) mostly from increased reflection from clouds back into space.[6]

Note: the Wikipedia content applies to this section only.

System types[edit]

Bag types[edit]

A very simple solar shower, effective in sunny regions, uses a black bag full of water hung in direct sunlight.

Simple hose and pipe systems[edit]

A very simple "system" can be devised by running water through some hose or pipe that is exposed to the sun, and connecting that to a storage vessel in a thermosiphon arrangement. A thermosiphon causes heated water to displace cooler water above it and as long as the heated water can continue upward, it will do so. The pipe/hose cannot have air present as this will halt the movement. There also needs to be a minimum of a ~4ft (1.2m) rise from hose to storage vessel. A loop can set up to circulate water from vessel to hose and back, which continues the heating process. Cool water is drawn from the bottom, circulated through the hose, and returns near the top of the vessel. As long as the siphon is not broken (air present), water can be dipped or drained out of the vessel for use. This is a simple open-loop system, meaning water enters and is removed for use from the system.

Another type can be called a batch heater, since it heats a volume of water using a thermosiphon, but uses a constructed solar collector to absorb the sun energy. Its limitation is that the tank is above the collector, which is on the roof or area exposed to the sun, so the hot water must be piped to the point of use, which costs heat loss. A special Communal solar water heating system has also been proposed using a batch heater.

More sophisticated systems[edit]

More sophisticated systems exist, some still employ an open-loop system. (by tapping into an existing water heater or some other vessel.) A solar collector in the sunlight with a pump and power source operate to either assist or supplant the existing water heating apparatus. The water circulates from the water heater tank to the exposed collector, and back to the tank, and this will continue to recirculate the water and heat it. A photovoltaic-powered low-volume circulating pump can be used in this system, avoiding the need for external electrical power. The more efficient and the larger the collector and the smaller the volume of tank storage, the faster the water will heat. The longer it operates, the hotter the water will become, until heat loss levels off the water temperature. This open-loop system works very well in climates where freezing temperatures are absent or rare. They can work in cooler climates using a system with drains to empty the water from the portion of the system subject to freezing. The drains can be manually operated or automatically thermostatically controlled. This type of system can be widely used to assist or supplant existing conventional water heaters.

Closed-loop systems are best in climates which freeze and reach lower temperatures, but are more sophisticated and therefore more expensive. In the closed-loop system a coolant, usually propylene glycol, is circulated through the collector then to a heat exchanger, where the heat absorbed is transferred from coolant to water. The propylene glycol remains liquid at much lower temperatures and will continue to absorb heat and transfer it to the water. The propylene glycol also remains in the system, hence the "closed-loop" name. The heat exchanger is either external to an existing water heater tank, or replaces the existing tank. A PV-powered low-volume circulating pump can also be used in this system.

These systems and associated technologies are arranged basically in order of cost, sophistication, and energy. The first of these is certainly an "appropriate technology" and could be used with minimal investment, and with guidance, can be used by practically any culture, regardless of perceived sophistication. The second type can be used in developing societies in original construction or retrofitted. Both types of installation can greatly reduce energy costs, GHG, and allow greater focus on other needs. The third, closed-loop system is more expensive, therefore more restricted to wealthier cultures, but its benefits are similar to the others. Based on per capita energy use, the more expensive systems can probably reduce more fossil fuel use than the others.

Evacuated tubes[edit]

Modern mass-produced evacuated tubesW collect heat even below freezing. The tubes themselves are best suited for mass-production, but the rest of the system is more flexible in its manufacture. Evacuated tubes use a vacuum sealed space to separate the collector tube from the outside elements. When solar radiation is absorbed by these collectors and converted to heat, the vacuum barrier prevents most of this energy from escaping. Essentially, this method operates in a similar manner to a thermos. The ability to contain captured solar radiation while preventing loss to the outside environment is what allows evacuated tube systems to continually heat water, even if the temperature outside of the system is frigid.

Disadvantages of Solar Thermal Energy[edit]

Solar Thermal Energy is only available wherever/whenever the Sun is visible.

Making you own solar thermal collector[edit]


  1. Solar Spectra: Standard Air Mass Zero NREL Renewable Resource Data Center
  2. Earth Radiation Budget NASA Langley Research Center
  3. Solar Maps NREL: Dynamic Maps, GIS Data, and Analysis Tools
  4. us_pv_annual_may2004.jpg National Renewable Energy Laboratory, US
  5. Homepage International Energy Agency
  6. Liepert, B. G. (2002-05-02) Observed Reductions in Surface Solar Radiation in the United States and Worldwide from 1961 to 1990 GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 10, 1421

See also[edit]

External links[edit]

Full Text Thesis[edit]