DogStar Solar's Alternative Energy Updates


Fine-Tuning a Solar Water-Heating System
July 30, 2009, 4:02 pm
Filed under: Solar Thermal

This landlord used data logging to adjust a solar water-heating system for maximum energy savings.

By Richard Reis, P.E.
Reis houseThe author pointed the collectors about 20 degrees east of south because of the building alignment. This slightly lowers the solar potential and means that most energy is gathered in the morning. PHOTO COURTESY RICHARD REIS, P.E.
Back in the 1980s, during the Carter administration, I installed a solar water-heating system in my house. That drain-back system still operates dependably and has paid for itself many times over.
After I bought a five-unit apartment building, it made sense to upgrade its plumbing to incorporate solar water heating. Each apartment had its own electric meter, so the tenants paid the bills for running their own lights and appliances. But the building had a house meter too, and as landlord, I was paying the utility bill for the washer, dryer, hall lights — and for a water heater that served all five apartments.
I studied the utility loads and determined most of the electrical energy from the house meter went to heat water. The solar system would be designed to displace most of that electrical demand. I solicited proposals from the few solar contractors in my area and selected one that offered a competitive and complete job, including design, permitting (no small feat in Montgomery County, Md.), materials, installation and warranty.
The contractor installed the system in March 2004. We pointed the collectors about 20 degrees east of south because of the building alignment. This slightly lowers the solar potential and means that most energy is gathered in the morning.

We went with a conventional, modern double- circuit system, with four flat-plate collectors, a pump, two storage tanks with built-in heat exchangers, a controller, a pressure gauge, a pressure tank and a bypass valve, with a glycol-water heat-transfer fluid in the closed primary circuit. The pump runs when the fluid temperature at the collectors exceeds the temperature at the bottom of a storage tank. We kept the original, conventional electric water heater. The solar system preheats water in the storage tanks, which is then fed to that original heater, which now functions as the backup water heater. When the temperature in the backup heater falls below its set point, its electric circuit kicks on. The hotter the water from the solar system, the less electricity the backup heater consumes. An anti-scald valve keeps outflow to showers and sinks safely below 120˚F (50˚C).

The following items were noted during initial commissioning:
• The contractor promptly resolved piping and venting problems.

• The backup water heater had been set to heat water to 130˚F (56˚C). We reduced that to 120˚F (50˚C).

• We reduced hot water use by replacing the apartments’ conventional sink aerators and shower nozzles with low-flow models, 1.5 and 2 gallons (5.7 and 7.6 liters) per minute, respectively.

• To improve appearance and extend life, we covered the black foam pipe insulation with white plastic.

• Because it cast shadows over the collectors, we removed an invasive mulberry tree. We planted new trees where they wouldn’t shade the solar system.

We installed an OnSet data logger (onsetcomp.com) to evaluate performance. I would use the data to help plan system improvements. The device records when the solar pump turns on and off, and it graphs both the temperature near the top of the storage tank and the electric current used by the backup heater. Since the backup heater presents a simple resistive load, multiplying current by line voltage yields power used. Integrating power over time for each day yields daily backup energy demand. The solar controller and pump also draw power, but it’s a negligible amount compared to what’s needed by the backup heater.

Chart 1Chart 1
Chart 2Chart 2

Chart 1, above, shows the performance of the system over several days in April 2008. On the days when the sun was shining, April 15 to 18, the solar pump ran, the storage tank temperature rose and the backup heater demanded little current. Chart 2 shows energy demand was lowest on those days. By contrast, April 13 and 20 were overcast. The pump ran less frequently, the storage tank temperature stayed lower, and the backup heater current and energy demand were higher. Energy use was high on April 14 because the two previous days were mostly overcast. Residents took their morning showers before sunrise, so the backup heater did the work.

Chart 1 shows that the storage tank temperature follows a consistent pattern on sunny days:
• Morning showers lower the tank temperature. [A]

• The solar pump turns on and the tank temperature rises. [B]

• Even when no water is drawn, the tank’s temperature drops slowly, indicating a low and constant rate of heat loss. [C]

• Other hot water use, such as late-night washing, also draws down the temperature. [D]
After measurement, the following issues were noted and corrected when feasible:
• The solar storage tanks lose heat (and energy) even when no water is drawn. These mechanisms may be at work:
– Conductive loss from the storage tanks. To reduce this loss we added insulation blankets. These recovered their cost in less than one year.
– Nighttime thermosiphon circulation may draw heat-transfer fluid from the hot storage tanks to the cooler collectors.
Further measurements will determine if this is occurring. If so, the addition of a one-way valve may resolve the issue.
• The factory set the controller to shut off the solar pump when the storage tank temperature exceeded 140˚F (62˚C). This set point was raised to prevent much higher stagnation temperatures in the collector plates, which might lead to degradation of the glycol mixture.
In addition:
• I reinforced the lightning protection by grounding the glycol pipes directly to the cold water supply line with heavy copper wire.
• Pipes and collector racks should be raised from the roof surface to permit rainwater and debris to flow freely to the gutters. We missed that issue, but it would be critical in a colder climate to prevent formation of ice dams.
The solar water heater, with the later improvements, has reduced the building’s electric draw from about 800 kWh to less than 300 kWh per month. After a state grant and the 30 percent federal tax credit, this system will pay for itself via energy savings in four years.
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Solar Water-Heating Economics
The economics of solar water heating work out to an attractive 51-month payback when measured by the avoided cost of electrical energy. Here’s the analysis:
1. The economic analysis of any capital investment starts with the cost of the proposed system, including components (collectors, storage tank, pumps, pipes, etc.), plus the cost of installation. To get a measure of those costs, I considered 10 web sites estimating the costs of complete and installed systems. The average cost was $7,360.
2. The total water-heating load can be computed in million British Thermal Units* (MBtu);
• The amount of hot water used per person (gallons/day): 33
• The number of people in the building: 8
• Days per year: 365
• Weight of 1 gallon of water: 8.33 pounds
• Required temperature: 120˚F
• Average temperature of incoming water: 58˚F
• Required temperature increase: 62˚F (120˚F – 58˚F)
Energy required: 24.88 MBtu per year
3. The amount of energy provided by a solar hot water system depends on the solar fraction — the ratio of solar heating to total heating required. I estimate this as 0.7— within the U.S. Department of Energy’s range for solar fraction. Thus the solar water-heating system displaces 17.42 MBtu per year.
4. For electric draw, convert the solar heat to annual kilowatt-hours, considering the efficiency of the water heater and the cost of electrical energy in kilowatt-hours.
• Kilowatt-hour per MBtu: 293
• Annual heater output required: 5,105 kWh
• Electric heater efficiency: 90 percent
• Annual heater input: 5,672 kWh
• Annual solar controller and pump usage: 244 kWh
• Annual net solar energy: 5,428 kWh
• Cost: $0.16 per kilowatt-hour (retail cost of electricity)
• Annual cost avoided: $869
Simple payback in years: 8.5 years
5. Consider government benefits:
• Federal solar tax credit: 30 percent
• Maryland solar grant: 20 percent (maximum of $3,000)
• Total grants: 50 percent
• Net cost: $3,680
Payback: 4.25 years, or 51 months. Payback times are longer when solar displaces natural gas water heating.

Photovoltaic Comparison:
The solar hot-water system displaces 5,428 kWh annually. To produce this much electricity would require about a 4-kilowatt photovoltaic system, costing about $30,000 — about four times the cost of the solar water-heating system without government benefits.
*A Btu is the heat required to raise 1 pound of water 1˚F.
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About the author: Rich Reis, P.E., is principal engineer of Conservation Engineering (conservationengineering.com). He can be reached at rreis[at]verizon[dot]net.
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