Deciding When Solar Hot Water Makes Sense
Solar hot-water collectors are common in net zero designs but not exactly a given. If the electrical output of other renewable energy systems (wind or PV) is sufficient, it probably makes more sense to heat water that way. This avoids the initial cost of solar collectors, including maintenance or repairs to the system, which can be considerable.
When to Add More PV or Invest in Solar Hot Water
This can be a vexing question. What pays the best dividends may not always be obvious, especially if the unpredictable habits of the whoever is living in the house can throw the best calculation right out the window. Consider this case, Habitat for Humanity house in Lenoir City, Tennessee built under the guidance of the Oak Ridge National Laboratory. Designers used computer modeling to look at solar hot-water options options and chose a system with 96 sq ft of collectors on the roof and a 200-gal. storage tank.
But the family of four used 20 gal. a day of hot water when 63 gal. had been predicted. This disparity was big enough for designers to wonder whether the $7,000 collector, plus the $1,300 tankless backup heater, was the best choice. It turns out that increasing the size of the PV system slightly and using and electric hot-water heater would have cost about $5,100 compared with the $9,300 total for the solar system that had been installed. Oak Ridge concluded that conventional wisdom may not be true in all cases and additional investigation into the comparison of solar water heating and PV investments may be needed.
Solar hot-water systems are a natural fit with net zero houses. They're capable of meeting much of the demand for domestic hot water, and in some instances can help with space heating as well. There are a variety of solar water heater on the market, some suited only for warm climates and some designed specifically to cope with subfreezing temperatures. No matter where you live, there's a solar collector out there that will work.
One way to think about the variety of collectors is to divide them into active or passive systems.
Active collectors use electric pumps to move a fluid (either water or anitfreeze) through the collectors and into a storage tank.
Passive systems, are simpler and less expensive, rely on gravity and convection to move water through the collector. There are no electrical components to break, and so these systems tend to be less expensive and more reliable but they're more susceptible to freezing pipes.
Basic Types of Collectors
Flat-plate collectors - are up to 4 ft. wide and 12 ft. long, consist of a shallow, insulated housing that encloses tubes for the water or heat transfer medium and plate that helps absorb solar energy. A sheet of glass covers the top of the collector. Fluid flows through the pipes, picks up heat from the sun, and is moved into a hot-water storage tank or directly into the domestic hot-water supply.
Evacuated tube collectors - These collectors consist of parallel rows of glass vacuum tubes. Inside, a fin absorbs heat from the sun and transfers it to a liquid medium. Evacuated tube collectors are sometimes specified in cold climates because of their high efficiency.
Thermosiphon collectors - Natural convection moves water through the collector without the need for pumps. As warm water rises into a storage tank, cooler water flows in the bottom to replace it. Some versions can be used in frost-prone areas, but these are typically found in climates that don't have freezing temperatures.
Making Passive Solar Work Where It's Cold
Water lines that freeze and burst have been one factor limiting the wider use of passive solar hot-water systems. The U.S. Department of Energy reports that the current practice of using insulated copper limits the use of these systems to the southern tier of the country and makes truly risk-free installation possibly only in Florida and along the southern California coast.
One promising piece of research involves pipes that can freeze and thaw repeatedly without damage. It also uses freeze-protection valves that protect water lines by circulating a small amount of warm water when the temperature dips toward freezing.
The National Renewable Energy Laboratory conducted tests on several brands of pipe made from cross-linked polyethylene (PEX). Some of them were able to withstand 400 freeze-thaw cycles without breaking; one brand could tolerate only 10 cycles. One problem with PEX is that it has an upper temperature limit of 210°F.
According to the DOE, if the amount of water used by freeze-protection valves, which is about 1,000 gal. per year is deemed acceptable and the pipe issue could be solved, the use of passive systems could be much wider.
Warm Weather Systems are the Least Expensive
There are lots of variables that affect the cost of system. An installed system in Florida, where long spells of freezing weather aren't an issue, should cost between $3,500 and $5,500 according to the Florida Solar Energy Center. On the other extreme, in Maine, an evacuated tube system for a family of four could top $10,000.
Federal and state incentives also lower the costs, sometimes substantially. Despite a depressed housing market, the National Association of Home Builders reported at the end of 2008 that increased federal incentives were helping Florida builders sell houses with both photovoltaic panels and solar hot-water systems at prices competitive with conventional houses in the same market.