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. That being said, the main disadvantage of solar thermal energy is that it is only available wherever/whenever the Sun is visible.

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.

Applications[edit | edit source]

2005 zaragoza system.jpg
Abri Belugas Evac Solar.jpg

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.

Background[edit | edit source]

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 | edit source]

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.

Types[edit | edit source]

Closed Loop Passive system in Parras de la Fuente, Coahuila Mexico

Solar hot water systems are designed to transfer the sun's solar energy to water. Finding the most efficient and effective solar hot water system for a given situation can be a challenging task. There are a number of key factors that need to be considered when choosing the most appropriate system configuration. These factors include, to a large extent, amount of solar insolation, climate, construction, installation and materials costs, location and accessibility of the system, amount of water needing heating, frequency of hot water use, availability of electricity, availability of materials, and skill level in construction.

The following classifications of systems are in three groups of two and one group of one unique system. These four main groups are:

  1. Open loop vs Closed loop.
  2. Active vs Passive.
  3. Uses a heat exchanger vs Does not use a heat exchanger.
  4. Batch system.

Any given system uses one characteristic from each group. For example, a system may be an active, open loop system which does not use a heat exchanger. Or another example, a system may be a passive, closed loop system which does use a heat exchanger. Some systems are much easier to make than others and people with a basic knowledge of tools and construction can easily make a functional system. If one desires to make their own system, this variability in complexity would influence what type of system is chosen.

Cost is another factor and each system configuration comes with a variety of different cost and benefits. The costs of any specific system can vary widely from country to country and region to region. Certain configurations using certain types of equipment are more efficient than others in specific situations. The following information gives an in depth look at these various ways of constructing hot water solar collectors.

Different types of collectors are also shown at the the end of this page as well as examples of different common solar hot water systems.

This page describes the various systems that are being used to heat up water with the sun. For a more general description of solar hot water visit the Solar hot water page.

Simple systems[edit | edit source]

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

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.

A batch heater.

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 | edit source]

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, 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 simple systems are certainly "appropriate technologies" and could be used with minimal investment, and with guidance, can be used by practically any culture, regardless of perceived sophistication. The open-loop systems can be used in developing societies in original construction or retrofitted, and as with the simple systems, the types of installation can greatly reduce energy costs, GHG, and allow greater focus on other needs. The closed-loop systems are more expensive, therefore more restricted to wealthier cultures, but their 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 Tube Collector on a tin house

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.

Solar hot water pools[edit | edit source]

Aaaannnnd ACTION! Tangible effects of the sun's energy.

Energy derived from the sun drives and sustains life on earth. So why can't it heat your pool?

Swimming pools... your skin tingles in anticipation of diving in clear cool water on those ridiculously-roastingly hot summer days. This precise moment of contact makes all the hassle of cleaning and caring for your pool worth it, does it not? Now if only the scorching days lingered longer so you could laze a bit more in your backyard tropical-wannabe paradise. Alas, the seasons don't listen to you, and inevitably fall, winter and spring attack your precious pool, chilling it to its tiled bones, making it completely unusable to you. Much of the year your pool sits, unused and unloved, like a dog eagerly awaiting the return of its owner, like a dormant daffodil bulb waiting for the snow melt, like a ... Just as surely as we weren't sent here to Earth to suffer, surely we can all get what we want. And if for you that includes a heated pool without the financial and environmental expenses of fossil fuels, then welcome to the wide world of solar hot water systems.

Related projects[edit | edit source]

References[edit | edit source]

  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 | edit source]

External links[edit | edit source]

Full Text Thesis[edit | edit source]

Discussion[View | Edit]

Some comments[edit source]

Here are some of my questions and comments on this page:

  1. The background seems to have an inappropriate amount of opinion.
  2. Should the page name be changed to Solar hot water basics, and Solar hot water just link to Category:Solar hot water.
  3. The photovoltaic references in the Energy From The Sun part seem very out of place.
  4. How is this Appropedia value added over a purely encyclopedic article that would be better placed at Wikipedia? This article should probably focus more on making it happen.

Thank you, --Lonny 19:35, 2 September 2007 (PDT)

Hey, all:

Some notes from a newbie who is not yet BOLD, and doesn't have the HTML/formatting skills to make quick improvements. First, on the technical side, "solar THERMAL energy" is incorrect since it is all sunlight of all wavelengths, and it is converted to infra red heat in the collector and transferred to the water. Collectors are black to absorb as much light as possible for this very reason. A white hose on the lawn has less energy conversion than a black one for this reason. A solar water collector works at the same conversion rate in the coldest weather as it does in hot weather for the same reason. It is not "thermal" until it is converted.

Second, functionally solar water is efficient and easy not for the reasons stated, but because the solar energy is free, i.e., NO FUEL COST. System costs can be very reasonable as well, which leads to....

Third, from a practicality standpoint, it should be stated in an appropriate place that solar domestic water heating systems are among the most cost-effective means of reducing utility and energy costs, and if fossil fuels are otherwise involved, reducing GHG is also cost-effective and direct. For this reason, a solar hot water system is the first thing people should consider purchasing or building. Single-family residences and buildings with <2-3 floors can be fitted with roof-top systems with little complexity due to pipe runs. Moer floors gets more expensive due to piping costs. Properly sized for user demand and sunlight availability, a system can supplant practically 100% of conventional water heating needs.

Fourth, I noticed a batch heater (type with the horizontal tank above the collector) photo at the top of the page and the same one at the bottom.

Fifth, to illustrate the installed extent of solar water, there is a photo of a European apartment rooftop with dozens of collectors. If I can find it, I will see about providing it.

Sorry at this point to appear to be on the sidelines kibitzing, but as I learn I will be able to act more directly to improve things.

David Messages done with sustainable energy, with Wind and Sun! 20:02 9 Aug 2008 CDT

Hi David,
Great comments. Below are responses to each of your points.
First: Good point, feel free to change the text as such. You can keep the link to the excellent, Practical Action Solar thermal energy page by placing it somewhere else more appropriate on the page. Or by using a pipe (|) to link using a different name, e.g. [[Solar thermal energy|Solar energy]] links to Solar energy.
Second: Functionally solar water is efficient for the reasons stated, i.e. it is more efficient than conversion because of the losses associate with conversion. That said, I agree that the effectiveness is even more a product of the incredibly cheap energy source of the sun (I usually differentiate between free energy and really cheap energy, solar power being really cheap because it is landing all day long but you still need to get it, whereas nothing is free... of course this is merely semantics). Please click edit on the introduction and change the text as you see fit.
Third: For any assertions, like stating what the first thing a person should consider is, citations are really useful. In addition, try to state where the statement holds true, such as "In the United States...".
Fourth: You can delete that photo by clicking edit and deleting the last image, which is written as [[Image:Solar_heater.jpg|thumb]].
Fifth: That would be great. Just make sure that you get permission (or that permission exists) to use the image under the GFDL or CC license (many photos on wikipedia and flickr use these open licenses). I took some nice photos of this in Morocco two months ago, but the camera was stolen before I could transfer them. I may have another image, that shows how extensive solar water heater installations can be, from somewhere else. Let me know if you do not find the image and I will look through my own.
Don't be sorry about kibitzing... lurking, then asking questions and starting to slowly engage is a good way to go. I hope that this feedback helps you proceed.
Please don't let your lack of formatting skills stand in your way. Most of what you need to know about formatting is at Help:Contents (and the rest under Category:Appropedia help). Please try editing and ask for assistance when you need it. All changes are stored under the history tab (click it at the top of a page to see all the changes), so we can always revert back if needed. Please do keep in mind that we are building global living library. Much more important than formatting, is truth, transparency and tone. Editorializing is great on the talk pages, but work on category pages should be referenced and scientifically valid. Thank you for helping us get closer to that.
Thank you, --Lonny 02:39, 10 August 2008 (PDT)(PS use 4 ~'s to get an automatic signature... you can set your signature in your preferences)

One extra note about licenses[edit source]

It's important to only use images with free licenses - which in CC terms means CC-BY-SA or CC-BY, and not anything with an NC or ND clause (i.e. "non-commercial" or "no derivatives").

Non-commercial license specifications make Richard Stallman sad ;). --Chriswaterguy 03:22, 10 August 2008 (PDT)

System types and circulation options[edit source]

  • Batch Solar Heaters: A batch heater is basically a metal tank inside a box that is glazed on the sun facing side and insulated on the other sides.

There are a bunch of designs for them here: They are pretty efficient and widely used. The tanks are somewhat resistant to freezing, but the plumbing between tank an house can freeze. People in cold climates often shut them down for the deep winter. In a climate like the one mentioned in the article you sent, they would probably be fine year round. Probably the biggest challenge is finding the tanks, but they are nice simple, low maintenance systems that don't require any power or control system.

  • Thermosyphon Collectors: This is the type of collector shown in the article you sent. Basically a solar collector that is mounted below a storage tank. When its sunny, the collector heated water rises up into the tank as its density decreases. At night the circulation automatically shuts down as the water in the collector cools and becomes more dense. They are nice simple systems in warm climates, as no heat exchanger is required. They do get more complicated in freezing climates and require a heat exchanger as in the one in your link. Some thermosyphon designs here: One thing I noted in the article is that they don't appear to specify what kind of antifreeze is used. There should be a warning against using ethylene glycol as it is poisonous. Propylene glycol is not poisonous and should always be used.
  • Drain Back Systems: Drain back system have the storage tank located below the collector and when the sun is not on the collector and the controller shuts the pump down, all of the water in the collector drains back into the tank -- this provides freeze protection in that there is no water in the collector when its cold. Normally these systems use a controller and a pump that work off line power, and this adds to the complexity, cost and maintenance. But, they are good reliable and simple systems. If the vertical distance between the tank and the top of the collector is relatively small, then it is possible to use a small DC pump directly powered by a small PV panel. This eliminates the need for a controller, as the PV panel only generates electricity when the sun is on it. PV panels and small DC pumps are available at pretty low prices these days.

Of all the systems mentioned above, the drain back and the thermosyphon with heat exchanger are the only ones that will work in a climate with hard freezes.

Mention in article, info from Gary from BuildItSolar KVDP 00:30, 12 July 2013 (PDT)

Images[edit source]

Images: solar water heating system.JPG and may be added to the solar hot water article; however they include "self-closing valves"; aldough I think this is possible to implement, not sure whether it allows the systems to be used as "appropriate technology" alternatives

Tank placement[edit source]

I think the placement (height) of the tank is important aswell. For example, The Zaragoza system placed the tank above the collector. This seems to be inefficient (pump needs to work harder, more energy loss). I think the basic concept/design can be kept, yet needs to changed a bit. Ie, by placing the tank lower -same height and size of collector-, see this image. Another main design flaw is that there is no metal between the pipes guiding additional heat to the pipes. This metal can be either a bend flat plate or fins; see and

Other improvements may be painting the box of the device black, correct tilt of the collector to the sun, depending on location (latitude) and time of the year, facing true south or north (depending on hemisphere), possibly more insulation on part, pipes, making the pipes to the boiler as short as possible, ... KVDP 00:54, 31 May 2013 (PDT)

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