The easiest way to use solar hot water is for heating “domestic” hot water. That is, all the water you use for washing (laundry, showers, dishwasher, and so on). For most households (2 to 4 people), it is possible to do 50 to 75% of the hot water heating with just 64 square feet of collectors and an 80 gallon storage tank. If you have a pack of kids or an extended family, you may need a couple more collectors, but still, this does not take up a huge amount of roof space.
The best place to put the collectors is on your south-facing roof, where there is no shade. Ground-mounts are also fine, but work best if they can be less than 50 feet from the house. That’s because long, underground, insulated pipelines are expensive. The best place for the storage tank is near the existing hot water heating system.
How Domestic Solar Water Heating Systems Work
We prefer to install closed-loop systems. These have 2 separate circulation loops, one to the solar collectors and one to the storage tank. The tank holds potable water. The collector loop holds a mixture of propylene glycol and water. Propylene glycol is a non-toxic anti-freeze, which is sometimes found as an ingredient in packaged foods. A controller turns pumps on to circulate fluid between the collectors and a heat exchanger, and between the storage tank water and the heat exchanger. When the collectors are hotter than the tank, the pumps turn on. The storage tank feeds hot water to the existing hot water tank. The existing tank will stay turned off when the storage tank temperature is higher than the set-point on the regular tank. For example, a typical gas-fired hot water tank is set at 110 F. After a couple of hours of good sun, the storage tank will be hotter than that, keeping the gas element turned off.
The solar collectors can raise the storage tank temperature as high as 150 F on a sunny day. These days, most houses have faucets, which can easily adjust the temperature of the water by mixing hot with cold water. If you do not have those kinds of faucets, or just for extra safety, we can put in a tempering valve at the hot water outlet at the tank. Then the temperature of the water coming out of the faucet can be limited.
Systems using propylene glycol will need to have the fluid changed once each 5 years. It’s also a good time to have the system checked out. They tend to just keep working for years on end, but like anything else, an occasional check-up will prevent problems later on. We have seen collectors as old as 30 years still working well. The most likely thing to need replacement (typically after 15 years) are circulator pumps. Luckily, those are very inexpensive (often less than $100). Storage tanks usually don’t last as long as the collectors because of hard water or excessive particulates in the water. This can be dealt with by flushing your tank each year, though most of us don’t really have the time or inclination to do that!
Heating a Building
There is a lot of interest in this, but it fairly difficult to do for a reasonable cost in the Northeast. To serve a decent proportion of your building heating, you will need a solar collector area equal to about 1/3 of the area to be heated. Thus, if you want to do 75% of the heating for a 2000 square foot house, you are going to need 2000/3 = 667 square feet of collectors. That amounts to twenty one (21) 4 x 8′ solar collectors! Since there can be many days without sun, you will need a large storage tank, usually 1000 gallons or more. The fundamental reason solar building heating is difficult is that, in the Northeast, when we need the most heating, we have the least amount of sun. This leads to the need for large collector areas. To have any hope of doing a significant amount of building heating for a reasonable cost you need to have a well-insulated building with radiant floor heating. Radiant floor hydronic heating is important because it does not require high temperature water. Radiant floors need 100 F water, and solar collectors can easily generate a large volume of 120 F water during the short days of winter. Getting 180 degree water for baseboard heating requires a significantly larger collector area and is therefore not as cost-effective.
So before you start looking for an alternative heating source, apply energy efficiency upgrades to the building. In New York, visit www.upgradeupstate.org to find out how you can get a free energy audit and recommendations. Improving insulation and applying some caulk might not be a snazzy as solar collectors, but will go a long way to reducing those heating bills!
If you are building a new house, make it an Energy Star house and install a radiant (hydronic) floor. Then a solar hot water heating system could do most of the heating. This has been done, even in Binghamton, NY!
Pool heating with solar energy is less expensive than any other kind of pool heating. In some places, most of the pool heating is being done with solar (Long Island, Florida, California). In the Northeast, the pool season is relatively short, but with solar pool heating, you could extend that season by a couple of months. This will give you more use of the (relatively expensive) pool. The collector area needed is about 75% of the pool area. Thus if you have a 20 x 40′ pool, you will need 0.75 x 20 x 40 = 600 sf of collectors. However, seasonal pool heating solar collectors are much cheaper than the glass-covered types used in domestic hot water heating or building heating.
Costs and incentives
Domestic solar hot water heating systems cost between $9000 and $15000 depending on size.
Building heating systems cost tend to cost tens of thousands of dollars, although for a new house, we have a chance of bringing the cost down to a more reasonable level.
In NY, you can get 25% of the system cost back as a state tax credit and 30% back as a federal tax credit. These credits can cut the system cost in half. Check with your tax accountant for the details of your tax situation.