Heating Water with Solar Energy
Solar water heaters are commonly used as heat sources for radiant floor systems in regions where an abundant solar resource is available. Normally, a large solar heated storage tank (with electric, gas, or oil backup) supplies hot water to the radiant system and most often provides for domestic needs as well.
Solar heaters interface well with radiantly heated floors because the large thermal mass common to radiant systems provides an excellent storage medium for the energy generated during the day. At night, this stored thermal energy is slowly released into the living space and a steady, even, and consistent comfort level is maintained.
The following schematics illustrate the components necessary for a solar based radiant floor heating system. There are several variations on the basic theme.
Solar Radiant System with External Heat Exchanger Solar Radiant System with Internal Heat Exchanger Solar Radiant System with the floor in an “open” configuration Open System with Solar Thermal and PV Direct
Sensor Placement
Unless you’re using a “PV Direct” solar thermal system (see above schematic), two sensors are required when a solar differential controller (solar relay) is used to trigger the pump sending fluid from the collectors to the solar storage tank. One sensor is placed on the collector at the hottest point of the header, i.e. as the fluid leaves the header and “returns” to the storage tank. The second sensor should be touching the tank itself. It should not be attached to the pipe leading to the tank’s solar heat exchanger.
Some solar storage tanks have a designated sensor port near the bottom of the tank. But, if no port is available, there is often an access panel near the bottom of the tank that will allow the installer to slip the sensor between the tank and the tank’s insulation.
Flat Plate Solar Panels
Flat Plate Solar Hot Water Panels Cutaway Flat Plate Solar Hot Water Collectors
A cutaway and real world application of “flat plate” solar collectors. The environmental benefits are only one of many reasons why solar water heaters are increasingly used in radiant heating systems. Simplicity, efficiency, and proven performance make modern solar equipment an excellent supplemental heat source.
Evacuated Tube Solar Absorbers
Evacuated tube detail Evacuated tube installed Combination Flat plate and Evacuated tube installation!
Because the evacuated tubes generate such high temperatures, they should be installed at the end of the solar circuit, effectively turning the flat plates into pre-heaters. Plumbing the tubes before the less efficient flat plates could actually cool the fluid coming from the tubes.
Most people are somewhat familiar with standard flat plate solar collectors. This collector is basically a highly insulated box containing a grid of copper pipes bonded to a flat black copper absorber plate. Special glass enhances solar absorption.
Evacuated tube collectors are an entirely different approach to solar water heating. Instead of many water filled copper pipes, these collectors use multiple vacuum filled glass tubes, each with a tiny amount of antifreeze hermetically sealed within a small central copper pipe. When heated by the sun, this antifreeze converts to steam, rises to the top of the tube, transfers its heat to a collector header, then condenses back into liquid and repeats the process.
Because heat doesn’t easily transfer through a vacuum, 92% of the thermal energy hitting the absorber plate stays within the evacuated tube and passes to the collector header. This is a huge advantage because a standard flat plate collector radiates much of its accumulated heat to the surrounding atmosphere like any other hot object.
The evacuated tubes are also completely modular. Although rarely necessary, one or more tubes can be removed and replaced without affecting the other tubes in the array. There is no actual liquid transferred from the evacuated tube to the collector header…just heat. Evacuated tubes also start absorbing heat earlier in the day than flat plates due to their convex design and the tiny amount of antifreeze within the tube is freeze protected down to.50 degrees below zero.
To minimize penetrations, this homeowner fastened 2′ X 2′ pavers to the asphalt roof with polyurethane construction adhesive, then used 3″ Tapcon screws to attach the stainless steel flat roof mounting kit to the pavers. Notice the highly insulated jacket around the supply and return lines. This “deck box” houses the two roof jacks (one for supply, one for return) that actually seal the two roof penetrations from the elements. The box is highly insulated, virtually weather tight, and provides clean and efficient access to the plumbing connections. Filling the deck box with fiberglass minimizes the heat loss.
How to build a solar heating system
Imagine you’re an inventor charged with the problem of developing a system that can heat all the hot water you need in your home. You’ve probably noticed that water takes a long time to heat up? That’s because it holds heat energy very well. We say it has a high specific heat capacity and that’s why we use it to transport heat energy in central heating systems. So can we devise a simple solar heating system using water alone?
Stand a plastic bottle filled with cold water in a window, in the Sun, and it’ll warm up quite noticeably in a few hours. The trouble is, a bottle of water isn’t going to go very far if you’ve a house full of people. How can you make more hot water? The simplest solution would be to fill lots of bottles with water and stand them in a row on your window-ledge.
Or maybe you could be more cunning. What if you cut the top and bottom off a plastic bottle and fitted pipes at each end, feeding the pipes into your home’s hot water tank to make a complete water circuit. Now fit a pump somewhere in that loop so the water endlessly circulates. What will happen is that the sunlight will systematically heat all the hot water in your tank (although it’ll never get particularly warm because plastic bottles standing on window-ledges aren’t that brilliant at collecting heat). But, in theory, you’ve got a working solar heating system here that’s not a million miles away from the ones people have installed on their homes. It’s very crude, but it works in exactly the same way.
Photo: The pipes that carry hot water to and from a roof-top, solar-thermal collector and into your house. Tight rubber seals (collars) keep cold rain from entering too! Photo by Warren Gretz courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
The parts of a solar-thermal hot-water system
In practice, solar heating systems are a little bit more sophisticated than this. These are the main parts:
Collector
This is the technical name for the big black panel that sits on your roof. Smaller homes (or ones in hotter climates) can get away with much smaller panels than larger homes (or ones in colder climates); typically collectors vary in size from about 2–15 square meters (~20–160 square feet). Not surprisingly, collectors work most efficiently on roofs that have a direct, unblocked view of the Sun (with few trees or buildings in the way). Broadly speaking, there are two types of collectors known as flat-plate and evacuated tube.
Flat-plate collectors
Flat plates are the simplest collectors: at their most basic, they’re little more than water pipes running through shallow metal boxes coated with thick black glass. The glass collects and traps the heat (like a greenhouse), which the water running through the pipes picks up and transfers to your hot water tank.
Photo: A typical solar hot water panel uses a flat-plate collector like this. Photo by Alan Ford, courtesy of US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
Evacuated tubes
These are a bit more sophisticated. They look like a row of side-by-side fluorescent strip lights, except that they absorb light rather than giving it out. Each tube in the row is actually made of two glass tubes, an inner one and an outer one, separated by an insulating vacuum space (like vacuum flasks).
Photo: An evacuated-tube collector. Note the gray manifold at the top and the white water pipe flowing through it. Photo by Kent Bullard, courtesy of US National Park Service and US Department of Energy/National Renewable Energy Laboratory (DOE/NREL).
How does it work? Briefly, we have a number of parallel, evacuated tubes (blue) that receive concentrated solar energy from parabolic reflectors either side (yellow), which they send to a combined heat-exchanger and manifold (brown), through which hot water (or some other fluid) flows from entry and exit pipes.
Artwork: A typical evacuated tube solar panel. Artwork from US Patent 4,474,170: Glass heat pipe evacuated tube solar collector by Robert D. McConnell and James H. Vansant. US Department of Energy, October 2, 1984, courtesy of US Patent and Trademark Office.
In a bit more detail: The inner tube is coated with a light-absorbing chemical and filled with a copper conductor and a volatile fluid that heats up, evaporates, carries its heat up the inner tube to a collecting device (called a manifold) at the top, where it condenses and returns to the bottom of the tube pick up more heat. The manifold collects the heat from the whole row of tubes and ferries it to your hot water tank. Unlike flat-plate collectors, evacuated tubes don’t let as much heat escape back out again, so they’re more efficient. However, since they’re a bit more hi-tech and sophisticated, they are usually much more expensive.
Artwork: A closer look at how an evacuated tube collector works. 1) The copper in the inner tube absorbs solar heat and evaporates the volatile fluid. 2) The evaporated fluid rises up the tube to the manifold at the top and gives up its heat. 3) Water flowing through the manifold picks up heat from all the tubes plugged into it. 4) The fluid condenses and falls back down the tube to repeat the process.
Hot water tank
There’s no point in collecting heat from your roof if you have nowhere to store it. With luck, your home already has a hot-water tank (unless you have a so-called gas combi boiler that makes instant hot water) that can be used to store heat from your collector; it’s a kind of hot water battery that you heat up at conveniently economic times (usually at night) ready for use during the day. If you don’t have a hot-water tank, you’ll need to have one fitted. The more people in your household, the bigger the tank you’ll need. A typical tank for a family home might be about 100–200 liters (30–60 gallons).
Heat exchanger
Typically, solar panels work by transferring heat from the collector to the tank through a separate circuit and a heat exchanger. Heat collected by the panel heats up water (or oil or another fluid) that flows through a circuit of pipes into a copper coil inside your hot-water tank. The heat is then passed into the hot water tank, and the cooled water (or fluid) returns to the collector to pick up more heat. The water in the collector never actually drains into your tank: at no point does water that’s been on your roof exit through a faucet!
How solar-thermal panels work
In theory
Here’s a simple summary of how rooftop solar hot-water panels work:
- In the simplest panels, Sun heats water flowing in a circuit through the collector (the panel on your roof).
- The water leaving the collector is hotter than the water entering it and carries its heat toward your hot water tank.
- The water doesn’t actually enter your tank and fill it up. Instead, it flows into a pipe on one side of the tank and out of another pipe on the other side, passing through a coil of copper pipes (the heat exchanger) inside the tank and giving up its heat on the way through.
- You can run off hot water from the tank at any time without affecting the panel’s operation. Since the panel won’t make heat all the time, your tank will need another source of heating as well—usually either a gas boiler or an electric immersion heater.
- The cold water from the heat exchanger returns to the panel to pick up more heat.
- An electric pump (powered by your ordinary electricity supply or by a solar-electric (photovoltaic) cell on the roof keeps the water moving through the circuit between the collector and the water tank.
In practice
Of course, it’s a bit more complicated than this! What if it’s winter and there’s no useful solar heat outside? You don’t want the solar system pumping cold water down into your home, but you still need hot water. And what if it’s really cold? You’ll need to stop your solar system from freezing up, so it would be useful to pump hot water from your home through it occasionally. That’s why a typical solar system will look more like this one, with two interlinked water circuits.
Artwork: A single-pipe solar heating system. Artwork from US Patent 4,191,329: Single-pipe hot water solar system by William E. Geaslin, Solartech Systems Corporation, published March 4, 1980, courtesy of US Patent and Trademark Office.
One (purple) pumps water through a solar panel as we saw above and down into a tank inside your home. This is connected to a second circuit (red) with a conventional hot water tank that can be heated by electricity, a natural gas furnace, or some other standard form of heating. On hot days, you effectively capture hot water in the purple circuit and then divert it around the red circuit into your home. On cold days, you can switch off the purple circuit using various valves or divert water from the red circuit through the purple circuit to stop it from freezing.
How good is solar thermal?
One of the most effective and efficient steps the government can take is to encourage the use of solar hot-water systems—a well-developed and relatively low-tech method for using the sun’s energy.
Larry Hunter, The New York Times (Op Ed), 2009
In pure efficiency terms, solar-thermal panels are over three times as efficient (50 percent or so) at harvesting energy as solar-electric (photovoltaic) panels (typically around 15 percent), but that doesn’t mean they’re three times better: it all depends what you want from solar energy.
If you live in the kind of family home where people are taking baths and showers all the time, especially in summer, solar thermal makes perfect sense. A decent system should be able to produce around half to two thirds of a home’s total, annual hot water needs (all your hot water in the height of summer and very much less in winter).
The obvious drawback of solar thermal is that it produces nothing but hot water—and you can only do so much with that; unlike photovoltaics, solar-thermal panels can’t help you heat your home or produce truly versatile, high-quality energy in the form of electricity.
The typical payback time for solar thermal (when your original capital investment has paid for itself in fuel savings) is about a decade, with a range of 5–15 years (depending on the cost of the fuel you’re saving, how much sun your home gets, and how much hot water you use).
Here’s a very rough comparison of the payback times for different types of green energy. It does depend entirely on what you’re installing, what you’re replacing, what existing fuel you’re not using instead, how much you used the old and new systems, and various other factors (such as tax incentives), so please don’t take the figures too literally.
The Cost of a Solar Water Heater
According to the U.S. DOE, a solar water heater costs, on average, around 100 per square foot of collector area. For example, if your system uses a solar collector that measures 4 feet by 10 feet (or 40 square feet), the average price is around 4,000. Depending on the complexity of your system, the price can range from around 50 per square foot to more than 400.
The U.S. DOE determined that water heating represents 14% to 18% of utility bills. equivalent to 400 to 600 per year. For a homeowner spending 600 on utility bills each year, a solar water heater that covers 50% of hot water demand would save 300. So if you purchased this system for 4,000, your payback period would be 13.3 years.
The above example is very simplified — the actual costs of a solar water heater can vary depending on the type of system and your home’s hot water needs. Solar collector can range from around 450,500 for small basic systems to over 13,000 for high-end units, according to EcoWatch.
Incentives and Tax Credits
Thanks to the Inflation Reduction Act of 2022, solar energy systems qualify for a federal tax credit equal to 30% of the total cost of your solar water heater. Considering the 4,000 solar water heater example above, the tax credit would be 450,200, which reduces the net system cost to 5000,800. Thanks to this incentive, the expected payback period drops from 13.3 years to 9.3 years in this example.
Depending on your location, additional solar incentives may be available from the state government or local utility company. You can check the Database of State Incentives for Renewables and Efficiency (DSIRE) to find rebates and other incentives by state.
Solar vs. Traditional Water Heaters
The main advantage of a solar hot water system is using a free energy source that reaches your home all year long. Solar collectors cannot heat water at night, but you can solve this by using an insulated tank to store hot water during the day.
Traditional water heaters run using gas or electricity and have the advantage of being able to heat water at any time. However, since these heaters depend on utility services, they can have a high operating cost. The U.S. DOE determined that the typical American home spends 400 to 600 per year on water heating.
There is also an environmental impact when using traditional water heating. Gas-fired water heaters release carbon emissions directly, and electric water heaters result in indirect emissions unless the local grid uses renewable energy.
Pros and Cons of Solar Water Heaters
Solar water heaters have advantages and disadvantages, like any piece of equipment. These systems can save you hundreds of dollars over time, but you should know their limitations.
Pros Service life of around 20 years Can help you save 50% to 80% on water heating costs Qualifies for a 30% federal tax credit
Cons Cannot heat water on demand without sunshine Upfront costs can be high Longer payback period than solar panel systems
Solar Hot Water Heaters Vs. Home Solar System
Solar water heaters are less common than they used to be. This is largely due to the drastic decline in the cost of solar panels, causing many people who would otherwise install solar water heaters to forgo them and heat their water with electricity generated from their own solar panels.
Solar water heaters take up precious real estate, and for a homeowner interested in producing their own solar-generated electricity, it may make more sense to maximize the space available and nix solar water heating altogether, buying solar panels instead.
However, if you don’t have the space for solar panels, solar water heaters may still be a great fit, as they take up far less room than solar panels do. Solar water heaters can also be a great option for those living in remote locations or as an environmentally friendly add-on for existing solar electricity generation. Modern electric water heaters are incredibly efficient and, when powered with solar electricity and paired with a solar water heater, will yield significant savings for your book and cut down your greenhouse gas emissions.
For many homeowners, the decision comes down to price. Solar hot water heaters can cost upwards of 13,000. To see how much a full home solar system would cost for your home, you can get a free, no-obligation quote from a top solar company in your area by filling out the form below.
FAQ: Solar Hot Water Heater
Is a solar water heater worth it?
Whether a solar water heater is worth it all depends on where you live, your needs and preferences, and whether you plan on installing solar panels. They have been losing ground due largely to the surge of home solar: The folks that would install them also want solar for electricity generation and often choose to eliminate solar water heaters that compete for valuable rooftop space.
If you have the space, a solar water heater will likely lower your water heating bills. Used in tandem with other renewable energies, it is still a great choice for nearly any application.
What is the price of a solar water heater?
A typical solar water heater system will cost around 9,000, with higher-end models reaching upwards of 13,000. Small-scale use heaters will be much cheaper, running between 450,000 and 3,000.
What are the disadvantages of solar hot water heaters?
The biggest disadvantage of a solar water heater is that it won’t work on foggy, rainy or cloudy days, nor at night. While this can be overcome with a conventional auxiliary heater, it is still a disadvantage all solar technologies share. Maintenance can be another turn-off. While generally requiring little maintenance, some need regular draining, cleaning and protection against corrosion.
How does a solar water heater work?
Solar water heaters circulate liquid through a solar collector — most commonly a flat-plate collector or tube collector — heating the liquid and sending it either to a tank for use or an exchanger, where the liquid is used to heat water for home use.
Christian Yonkers is a writer, photographer, filmmaker, and outdoor junkie obsessed with the intersectionality between people and planet. He partners with brands and organizations with social and environmental impact at their core, assisting them in telling stories that change the world.
Christian Yonkers is a writer, photographer, filmmaker, and outdoor junkie obsessed with the intersectionality between people and planet. He partners with brands and organizations with social and environmental impact at their core, assisting them in telling stories and spreading verifiable information that change the world for the better. Christian aims to have a sizable impact on the world helping foster a sustainable relationship between humans and the planet.
Irma joined the EcoWatch team in August 2015. She holds a Master’s degree and Bachelor’s degree from the E.W. Scripps School of Journalism at Ohio University in Athens, Ohio. She is a member of Kappa Tau Alpha – a national honor society of journalism.
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