How do you heat a house with solar power?

In many climates, heating is the largest energy consumer in your home. If you want to use solar power to do this, there are a few different options.

Photo of evacuated tube solar thermal collector.

If you live in a northern or midwestern state in the US, space heating is probably the largest energy consumer in your home.

In most climates in the US, central heating is accomplished with the use of fossil fuels. Natural gas is most common, but oil heating is still used in many regions. Even coal hasn’t completely disappeared for home heating use.

Basically, there are two types of solar technologies you can use to heat your home: solar thermal, which uses sunlight to directly provide heat, and solar photovoltaic, which turns sunlight into electricity that can be used to power your heating system.

There are pros and cons to both technologies, and the best choice for you will depend a lot on the type of heating system that you have. Read more to understand the different options and how they work.

Heating your home with solar thermal collectors

Solar thermal collectors directly turn the sun’s energy into heat, where it’s collected and tranferred to a hot water tank in your home.

Unlike solar photovoltaic panels which use semiconductors to turn sunlight directly into electricity, solar thermal collectors are relatively simple devices - simple enough that it’s possible to build your own with common materials. They work by using a dark colored collector to absorb heat energy that gets transfered to a fluid in pipes. The heated fluid travels along a loop to a heat exchanger that dumps the heat into a hot water tank. That can supply the hot water needs for a home, or the hot water can be used for other purposes, like radiant heating.

Types of solar thermal collectors for the home

There are different designs for solar thermal collectors. Utility-scale power plants, like the Ivanpah solar plant in the Mojave Desert, use fields of mirrors to reflect sunlight at a central collector, which uses the massive amount of collected heat energy to drive steam turbines.

Ivanpah is huge, with a nameplate capacity of around 400 megawatts. This type of design is possible in a desert where space isn’t a concern, but a homeowner needs something more compact.

That’s what a flat plate collector is. It’s simply a set of fluid-filled tubes inside a black colored collector with a glass cover. The glass cover allows sunlight to heat the pipes inside and also helps trap heat inside the collector. Depending on your climate and the amount of heat energy you need, multiple collectors can be connected together so that the final temperature is very high - even above the boiling point of water.

Example of a solar flat plate collector.
Example of a solar flat plate collector. (credit)

This is the type of solar thermal collector that is used for homes. The nice thing about these collectors is that they are low profile, and each one takes up about the same space as a photovoltaic panel. There are two basic types:

  • Flat plate collectors consists of copper tubes inside a glass-covered panel. Fluid moves through the copper piping, heating up before exiting the collector.
  • Evacuated (vacuum) tube collectors work similarly, except instead of a continuous copper tube, the collector is made up of several individual collectors that are vacuum sealed, like thermos bottles. The pipes are also exposed directly to the sun, rather than sitting inside a collector box. This type of collector is a better choice for cold climates because less heat energy is lost to the atmosphere. The photo at the top of this article shows what an evacuated tube collector looks like.

Hot water from the sun

The heat from either of these types of collectors is dumped into a hot water storage tank. A couple collectors may be all you need to satisfy the hot water needs for the average home. If you have larger hot water needs, such as for a pool or hot tub, you will need more collectors.

Hydronic radiant floor heating

If you have a hydronic radiant in-floor heating system, your solar hot water can then be used to heat your home. Many hydronic systems use the same system to heat both the fluid for the in-floor heat and your hot water supply. This is the best case because it means you can use solar for all of your heating needs. If your system has separate heating systems for hot water and the radiant heat, you will have to make some decisions about how to integrate a solar thermal system. In most cases, your home heating will be the bigger energy hog.

Hot water and steam radiators

Traditional radiators are similar to in-floor radiant systems in that they circulate hot fluid (water or steam) through pipe to radiators in your living space - the difference being that radiators are units placed against a wall or floorboard, rather than hidden in the floor. This type of system can also be integrated with solar thermal. However, many homes in the United States that use radiators are systems that are decades old, so integrating solar thermal may required additional upgrade work.

Forced air heating

It’s even possible to use a solar thermal system to heat a home with forced air heating. This works with the use of a heat exchanger in the airflow of your forced air system. The heat exchanger has metal fins that allows the collected solar heat to be dumped into the air, where it’s circulated into the ductwork.

This involves retrofitting into an existing system. The heat exchanger will often be installed in the cold air return plenum due to space availability.

Standalone heat exchanger

One last option is to run the solar heated fluid into a standalone heat exchange in your living space, such as a baseboard or wall mounted radiator. This has the advantage of not needing to modify any existing systems, but it also means having a separate heating system. This could be a good choice for a small house without an existing heating system, or if you want to add some heat into a space that isn’t already heated, such as a garage or basement.

Heating your home with a solar photovoltaic system

Solar photovoltaic panels can be used to heat your home as well. In many ways, this is a preferable approach because it’s a mechanically simpler approach, especially if you already use electricity to heat your home. With electric heating, there’s no separate system that carries heated fluid into your home, and no heat exchangers that need to be mechanically integrated.

With photovoltaics, you simply add as many panels as you need to supply the extra power for your heating system. This can be a disadvantage, however, if you have limited roof space. If that’s the case, you would want to consider using higher efficiency solar panels.

The one main disadvantage of solar photovoltaics for heating is lower efficiency compared to solar thermal. While solar thermal efficiency varies greatly with the climate (solar thermal collectors lose more heat to the environment in colder climates), in the best case scenario they can be more than 70% efficient.

The best solar photovoltaic panels, on the other hand, have a nameplate efficiency of about 24%, and reach less than that in real-world conditions.

That said, it may be lower cost to add more panels to an existing photovoltaic system than installing and maintaining a separate solar thermal system. This also makes for more flexible expansion and system updates in the future.

Air source and ground source heat pumps

Heat pumps extract heat from the outside environment and dump it into your home. They do this by using a compressor and a coil filled with refrigerant. Mechanically, this is similar to your refrigerator or an air conditioner. In fact, heat pumps usually are also able to work as air conditioners in the summer, making them an even more useful appliance for your home.

One big advantage of heat pumps is that they are very efficient heating devices - much more so than electric resistance heaters, like space heaters.

Generally speaking, there are two types:

  • Air-source heat pump. This type works by extracting heat energy from the air by using a fan to blow air over a compressor coil. The air warms up the coil, and then a compressor then squeezes the refrigerant liquid inside the coil, causing it to heat up. A second fan blows air over the heated segment of the coil, extracting the heat which is then used to warm up your house. This works even in very cold weather: some cold climate heat pumps will work down to 0°F. If you’re wondering how it’s possible to extract heat from cold air, think of a deep freezer. The average deep freezer chills your food down to around 0°F, but the coils at the back of the freezer are still warm. This because heat energy is still present in the air.
  • Ground-source heat pump. Instead of extracting heat energy from the air, a ground source heat pump relies on a refrigerant loop buried in the ground. A few meters below the surface, the temperature of soil doesn’t fluctate very much, even in the depths of winter or the hottest days of summer. The more moderate temperature in the ground means that this type of heat pump is even more efficient than an air-source pump. The downside is that ground-source heat pumps are more expensive to install because of the work to bury the refrigeration coil in the ground.

How much electricity does a heat pump use?

The amount of electricity that an electric heat pump varies greatly with its size (rated in tons or BTUs), and also differs by manufacturer and model. To find out how much electricity your model uses, check with the owner’s manual. You will want to add up the electricity usage that is listed for the compressor motor, compressor fan, and outdoor fan.

Keep in mind that the number of amps listed in the manual for the motor and fans are just nominal ratings, and the actual electricity usage will differ depending on the real world load that the unit is under.

Still, to help size your solar photovoltaic system, the numbers you get from your heat pump’s manual are a good enough estimation. To help you out, we took a look at some split system heat pumps from major manufacturers (Rheem and Goodman) and averaged the electricity usage by unit size. We’ve rounded these to emphasize that these are just approximate numbers.

Air-source heat pump size (in tons)Approximate maximum wattage

How many solar panels do I need to run my heat pump?

If you already have a heat pump, just look at your electric bill during the heating months to see how much more electricity compared to months that your heat pump doesn’t run.

If you’re planning to add a heat pump system in the future and want to estimate the electricity usage, you’ll need to do calculations to estimate the correct size of heat pump and its run time.

The maximum wattage of a heat pump only tells you how much electricity it uses when it’s running full throttle. How much electricity it will use to keep your house warm depends on the temperture outside, how well your house is insulated and air sealed, and what the square footage of your house is.

Calculating your whole house heating or cooling load is complicated, and requires an HVAC professional to perform a Manual J calculation. This calculation is hard to do correctly, so be sure to choose a contractor with a lot of experience in this area.

Once you have the calculation done by a qualified professional, you’ll have a good estimate of the correct size heat pump for your house and climate, and the approximate run time to expect. This will give you the number of kilowatt-hours that your heating system will use, which in turn will tell you how many solar panels you will need (by using our solar calculator to estimate the power generation for your climate and roof).

Electric resistance heating

An electric resistance heater uses electricity to heat up high resistance wires, just like in a toaster or hair dryer. Small space heaters, the kind you might place beside your desk in a cold office, use electric resistance.

Because this type of heating is very inefficient, it’s rarely used for whole house heating, except in climates with few heating degree days.

A small space heater uses about 1,800 watts. To see how much power your electric furnace uses, check the label. Read your electric bill during your heating months to estimate how much additional power you’re consuming to heat your home.

Lots of options for heating your home with solar

As you can see, there are a lot of different ways that you can heat a home. Fortunately, solar power is a completely viable way to provide that heat, no matter what type of heating system your house currently uses.

Both solar thermal and solar photovoltaic technology have pros and cons. Which you choose depends on the type of heating (and cooling) system that you want to use.

These choices can be technically complicated, so we recommend working with an experienced HVAC professional to correctly size your heating system and recommend the best way to integrate either a solar thermal or photovoltaic system into your HVAC mechanical system.

One service that The Solar Nerd provides is getting mulitple quotes from qualified solar contrators. This service can help you find an experienced professional who will properly install your solar thermal or photovoltaic system, and choose a system that is correctly sized to provide your heating needs.

Further reading

#Heating #Heat Pumps #Solar Thermal

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