ASK THE EXPERTS: Which Solar Application?

Intermediate
The roof space required for solar pool collectors is equal to about half of the pool’s surface area.

I’ve moved into a 1960s house in northwest New Jersey that has a modern, efficient oil heater for water and space-heating (using radiators). It has six rooms downstairs and two upstairs. There is also an unheated “three-season room” downstairs that I would like to convert into an office. And there is an existing swimming pool with no heater. We use well water.

I understand I can’t replace the oil heat entirely with solar. So I’m wondering what the best energy-efficiency improvement is? Should I supplement water heating with a solar water heating system? Should I just set up a PV-powered radiant heating system for the three-season room? Should I just solar-heat the pool, which seems the most efficient use of solar energy? Or should I forget heating and use PV electricity to reduce my dependence on the local utility? Payback time is a consideration. (Note: There is also a propane-fired whole-house generator in place.)

Bob Doyle • Ringwood, New Jersey

It is difficult to give advice on what is optimal economically since it is so dependent on local incentives. Here are some guidelines that may assist your decision-making.

•           Solar pool heating systems don’t require incentives to have a good return on investment (ROI). The systems are so inexpensive that they are the No.1 solar heating system installed in the United States. They require about half the pool surface of pool collectors, so available roof space can be a factor.

•           Grid-tied PV systems have dropped in cost dramatically and are eligible for the 30% federal tax credit, and with net metering potentially offer a good return on investment. Net metering allows building up a credit for any excess PV generation, which can be used during times of less production. Local incentives and net-metering rules for each state can be researched at dsireusa.org.

•           A solar heating system to address either domestic hot water or space heating is probably the least cost-effective system to deploy if your oil burner is truly a high-efficiency appliance and the cost of oil remains at the present level for the next few years. Rising oil prices could change this. But it is difficult to have a good ROI with a solar water heater displacing the energy of a high-efficiency appliance burning a low-cost fuel.

•           Environmental aspects should also be given careful thought in the solar installation decision-making process.

Chuck Marken • Home Power Thermal Editor

Comments (3)

Fred Golden's picture

You did not mention how you cool the home. Heat pump water heaters have become very popular in my area, (Portland OR) where a rebate almost covers the cost of the unit. Changing from oil hot water to heat pump should provide some savings, and allow shutting off the boiler for a significant part of the year. I would recommend the GE 80 gallon heater, rated at 64 gallons of hot water for the first hour, while in the heat pump only mode. Lowes has a great price. By feeding it through the oil heater, in the winter you can shut off the heat pump when you would be running the oil boiler because your basement is to cold. It will cool and dehumidify the room it is located in, such as a garage or basement.

Also replacing all electric heat with a 20 SEER ductless heat pump offers significant cost savings. In Oregon there is a $1,500 rebate program that covers much of the cost. By installing a 24,000 Btu heat pump in the living room, you can cool a large portion of your house, and provide heat until the days are under 50F for a significant part of the home. Only needing to run the boiler for the colder parts of winter.

Likewise a properly sized ductless heat pump for the office, can cool not only that room, but the nearby rooms, and provide economical heat in the winter. Ductless heat pumps will work well down to 20F, and still provide some heat (about 50% of rated capacity) at 0F outside temp.

Heat pump water heaters have saved municipal pools significant energy costs, especially of changing from a propane boiler to electric heating. With SEER around 17 on a warm day, they use very little cost per 100,000 Btu's of heat generated.

While a pool solar system will only be used about 3 months per year in your area, a 3 KW grid tied solar system can be used 350 days a year when not covered in snow. If tilted steeply enough, it will shed the snow quickly as the day warms up. Then the PV electric can convert to heat with a heat pump.

If you compare the cost to generate 100,000 Btu's of heat with oil, propane and electric resistance heat, you can see how much a heat pump will likewise consume to make 100,000 Btu's.

Oil will take about 1 gallon of fuel. 135,000 Btu's per gallon X about 70% efficient.

Propane at 95,000 Btu's per gallon would take about 1.1 to 1.3 gallons of fuel, depending on the efficiency of the heater.

Electric resistance heater despite being 100% efficient will only make 3,414 Btu's per KW or take around 30 KW.

The heat pump with 20 SEER will take 5 KW, because it generates 20,000 Btu's of heating or cooling per KW of power consumed. 16 SEER would make 16,000 Btu per KW, or take about 6.25 KW.

It is common to find ductless heat pumps in the 20 - 26 SEER range.

Jason Szumlanski's picture

I should also add that the best energy efficiency improvement for a pool home is often a variable speed pool pump to replace a standard single speed or 2-speed pump.

Solar pool heaters are often added as a way to improve enjoyment of a pool, and not always as a replacement for an existing heater, so it isn't strictly an energy efficiency measure. Many (most) pools in Southwest Florida are not heated, so a solar pool heater addition, while efficient relative to gas or electric heaters, is not technically an energy efficiency improvement.

Jason Szumlanski's picture

Thank you for bringing solar pool heating to light. It's a technology that is not talked about enough, and is extremely popular in some areas of the county.

However, I take issue with the sizing recommendations in this article. While the questioner is in New Jersey, in Florida where pools are used virtually year-round, especially in Southwest Florida where I operate Florida Solar Design Group, sizing of systems needs to be approximately equal to the pool surface area for a well oriented system, not 50%. The 50% factor is for climates where pools are only used part of the year, and the goal is to boost the pool temperature during good ambient swimming conditions only. To achieve customer satisfaction here, a 100% factor is far more typical. Fortunately, swimming pools are often smaller here, so the total system size is not dramatically larger. In fact, the 8-panel system pictured is pretty typical.

However, the pictured solar panels, which are terra-cotta in color, are extremely poor performers relative to the far more typical black panels. With a non-black panel your 50% factor should really be about 75%, and my 100% factor would be 150% to provide similar heating performance.

The blanket statements in the answer are too simplistic, and prospective buyers should seek the advice of a local solar pool heating professional for a satisfying result.

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