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Solar Collectors:

Three SunEarth EC32, 4 by 8 ft.

Appliance Module:

X-10 Powerhouse

Faucet:

Farthest from hot water tank

Drainback Tank:

Whirlpool, electric heater (disconnected), 19 gal.

Solar Storage Tank:

Rheem Solaraide 120 gal., integral heat exchanger

Backup Heater:

Crown, 50 gal., indirectly heated by boiler

Faucet:

Farthest from hot water tank

Drainback Tank:

Whirlpool, electric heater (disconnected), 19 gal.

Solar Storage Tank:

Rheem Solaraide 120 gal., integral heat exchanger

Backup Heater:

Crown, 50 gal., indirectly heated by boiler

- Other Alternatives for Making SHW Systems More Efficient

Water heating designs have been pretty solid for decades, but it's always possible to make improvements to the systems. Andrew made some modifications that work for his situation—here are a few others to consider.

Heat loss from a backup water heater can cause unnecessary fossil-fuel usage if the solar storage tank is hot enough to supply the hot water but there is no demand to move the solar-heated water to the water heater. With the tanks normally piped in series, this can happen with intermittent hot water usage. Andrew chose to equalize the two tanks and combine the equalization with a hot water recirculation system (more on that below). Another possibility would be to circulate between the two tanks, rather than throughout the household's water delivery system.

Extra insulation on the water heater can help. Many older water heaters have poor insulation, and wrapping the tank with extra can cut standby heat loss considerably. In areas where hard water isn't a problem, a tankless water heater that will modulate its output with the incoming solar-heated water solves the standby loss.

Gas, propane, and fuel-oil water heaters have an uninsulated flue pipe running up the center of the tank and through a home's roof to dispose of the combustion products, which contributes to heat loss. If your backup water heater is fossil-fuel fired with a conventional flue pipe, consider a highly insulated electric tank as an alternative. At this time, the price of oil is low, but a few months ago the price of propane and fuel oil made electric water heaters attractive. Electric tanks are always a better backup type of tank since they don't have a flue to add to standby heat loss.

Hot water recirculation systems are becoming more popular in upscale residential construction. They've been standard in larger buildings for decades. They provide instant hot water no matter how far the tap is from the water heater, which can reduce the amount of wasted water flowing down the drain while waiting for the water to heat. However, this convenience and water savings has an energy cost. The power that the pump uses is one cost and the heat lost through the pipes as it endlessly circulates through the system is the other cost. Timers and devices like Andrew designed mitigate both of these losses considerably, but it's always a tradeoff. Where water is abundant and energy is precious, these systems are of questionable value, except for convenience. In areas where energy is more abundant and water is scarce, the recirculation systems can be beneficial as well as convenient.

Recovering heat from a drainback tank is a novel idea and could be beneficial. However, it would be best not to route the water recovery system through the collectors at night. To keep pumps running as cool as possible, locate them on the cool side of the heat exchanger—where the heat has been pulled out already.

—Chuck Marken, Home Power Solar Thermal Editor

Unfortunately, no differential solar controller I'm aware of, including the dual-output models, will allow this process to be automated. What's needed is a special, additional controller that will turn on the circulator for a few minutes when the drainback tank's temperature is a few degrees hotter than the solar tank's bottom (and the collector is not too cold), until the automatic function of the main differential controller takes over. Since no commercial solution appears to be available, I hope to design one myself. (Contact me via e-mail for more information.) For now, the only way to extract the trapped drainback heat is to manually set the controller.

Performance

Historically, my family used about 550 gallons of oil for heating water, split almost evenly between the nonheating and heating seasons. The solar hot water system should offset 80% to 90% of this during the nonheating season, and about 25%, or 56 gallons of oil, during the heating season. Annually, the system offsets about 250 gallons of oil. At $4 per gallon, that's a savings of about $1,000 per year, which covers most of my loan payments for the system.

Calculating the savings from the IHWC is more complex. During the nonheating season, I estimate my boiler (which burns 3 gallons of oil per hour) fires six times per day to compensate for water heater tank cooling. It fires for about 3 minutes each time, for a total of 18 minutes, burning about 0.9 gallons of oil per day. From June to September, the solar hot water system will supply all our hot water on clear days. According to meteorological statistics, there are likely to be about 32 completely cloudless days in my area for that time period. This means the IHWC and associated components are likely to save a minimum of 28.8 gallons (32 days x 0.9 gallons) of oil per year, recouping costs in about three years.

To calculate the potential savings from my system that extracts heat from the drainback tank, it's necessary to estimate the number of times a cloudy day follows a clear day from June to September. I figure that a cloudy day comes after a clear day for a total of 32 days during that period. Since the drainback tank stores the equivalent of about one-tenth of a gallon of oil (as discussed above), an automatic system would save 3.2 gallons per year. This estimate does not include the reduced losses or increased efficiency savings discussed previously.

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Andrew Goldbaum ([email protected]) has an MBA, and is an electrical engineer, programmer, and director of software development for a small New York-based defense contractor.

Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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