Active Solar Thermal Systems
Using Solar Energy for Water Heating and Space Heating (and maybe someday cooling)
SDHW (Solar Domestic Hot Water)
SDHW is one of the most cost effective ways for the average homeowner to start using solar energy. For around $3000 to $5000 dollars (not including installation), you can purchase your very own solar water heating system. This may sound like a lot of money, but depending on your utility rates and the amount of hot water your family uses, the system can pay for itself in ten years or less (especially if there are rebates available in your area). The life expectancy of a typical system is over 20 years, so for much of it's lifetime the system will be saving you money. Compare this to say, a fancy granite countertop or decorative wrought iron all over the house, which can cost just as much or more and are simply pretty to look at. I'll take the solar system, thanks.
In our area, the majority of systems are indirect "closed loop" systems. This means that a separate fluid, usually a propylene glycol/water mix circulates through the collectors and then through a heat exchanger, where it transfers heat to the potable water in a solar storage tank. This provides freeze protection in cold climates. The pump (or pumps) for the system can be powered by a photovoltaic (solar electric) panel, which means that the system can operate without power from the utility grid. The heat exchanger should be double walled, to reduce the risk of contaminating the household water supply with the antifreeze. The antifreeze should be non-toxic (propylene glycol) - never use ethylene glycol (car antifreeze).
Heat exchangers can be mounted on the outside of the storage tank or integrated into the storage tank itself. Personally, I'm partial to side arm heat exchangers like the AAA Solar Quad Rod. These heat exchangers mount on the side of the storage tank and operate via natural convection. This means there is only one pump in the system, on the solar side. It also means that you can use a standard electric water heating tank without the heating elements. Any time a system can use off the shelf items it's likely to be less expensive. Plus, should the heat exchanger even develop a leak (not common, but it does happen occasionally), it can be swapped out while leaving the tank itself (and all the associated plumbing) intact. If an integral heat exchanger fails, the entire tank and heat exchanger both need to be replaced.
Although I have seen simulated studies that indicate that pumped systems (systems that use a pump to circulate the water through the heat exchanger) have a higher solar fraction (gather more of the available sunlight), side by side testing at my old employer, Public Energy Systems indicated that side arm natural convection systems provided better overall performance. Because the flowrates in these systems were lower than the pump systems, the water in the storage tank would become more stratified, concentrating the hottest water at the top of the tank. This means that rather than having an entire tank at 90 degrees, for instance, you could have the top "layer" of the tank at 120 - 130 degrees while the lower part of the tank was cooler. When there is a draw on the system, water is drawn from this hotter top layer, requiring less supplemental heating by conventional sources. Additionally, one less pump means one less piece to break down or require maintenance and a lower power requirement to operate the system, either through AC or PV.
SDHW normally provides between 40% to 60% of a household water heating load, possibly higher in warmer climates and during summer months. We have some used collectors that we plan to install for water heating, but the "balance of system" cost (the rest of the pieces) is pretty high and it just hasn't been in the budget yet.
Solar Space Heating
Solar thermal systems can also be used for space heating. For liquid collectors, the concept is similar to SDHW on a larger scale. Heat is gathered into a storage tank, then drawn off as needed through a heat exchanger submerged in the storage tank. Generally these systems combine space heating and water heating, so they are more complex (and more expensive). During warm weather, you must provide another load (such as a pool) or a thermal shunt to remove heat from storage and "dump" it outside the building to prevent overheating. Another alternative is cover a part of the array, but this is messy and requires more homeowner interaction.
Air collectors can also be used for space heating. Their biggest advantage is that they are easier to install, and if the hot air leaks all that's going to happen is that the system will be less efficient (you won't accidentally flood the house). They are usually an easier option for the do-it-yourselfer. I don't personally have any experience with these systems.
Solar Cooling
Although their are no commercial solar cooling systems available on the market for the homeowner (to the best of my knowledge), a team of researchers at the University of Florida has combined two thermodynamic cycles to create a cooling affect using solar heating that can be used for air conditioning or refrigeration. This technology sounds promising to me because they say they can use standard collectors, plus it could also potentially be used with geothermal systems or to improve powerplant performance. A crossover technology (one that can be used in more than one industry) stand a better chance of being developed. This is the first time I've seen solar cooling approached from this angle. The industrial/commercial systems I had seen in the past use adsorption chilling. As with nearly all solar technologies, up front costs are the biggest obstacle to implementation.