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Example Solar Hot Water
OBJECTIVE:
To reduce the cost of heating water by using solar collectors to heat water in the existing hot water heater tanks.
ELWA Hospital has 11 water heaters of 4.5 kW each, for a total of 49.5 kW when all the water heaters are on. This is a significant portion of the hospital load on the generators and electricity cost.
Solar hot water collectors with a solar-powered circulating pump would reduce the total load of the hospital, and reduction of electricity cost, by providing a significant portion of the hot water needs. In some cases, the solar-heated water could provide all the hot water required.
A smaller system using identical panels has been operating on one of the campus residences since 2015 and provides most of the hot water for the home. Output in rainy season is significantly reduced.
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| Example four panel solar hot water system. Quoted system includes parts for 10 arrays of 8 panels. |
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| Mounting frame for 8 solar hot water panels on ELWA Hospital roofs. |
ROI of 1.5-2.5 years (based on 2019 Quotation) with a budget of $29,000
- Non-penetrating roof mounts for collectors
- Collector frame cantilivered out from roofing truss (truss is 12ft above ground)
- Direct Solar-powered circulation pumps1)
- Only circulates water to hot water tank when there is sufficient sun to heat water
- Tank adapter from Heliatos – Specific for Rheem Marathon PVC hot water tanks
ROI Notes
Without installing water and electric meters on each hot water heater it is difficult to estimate actual cost and savings of heating water.2) The water heaters are high efficiency and are well-insulated, and would preserve solar heated water for long periods (24 hours or more). Assuming that the solar panels heat the water significantly above the thermostat setting (they are capable of heating water to about 180F3)), the thermostat may not switch on the electric heater for many hours after the solar panels are no longer collecting heat, even with moderate hot water use.
The largest user of hot water is the hospital laundry; as such it may be worth installing two systems on that water heater – one on the east side to capture morning sun, and one on the west side to capture afternoon sun.
See the linked Google Sheet for detail of the ROI calculations and a breakdown of the materials and installation costs.
| Energy Savings Estimate: | Heat, BTU/H | 4 hr Solar Day |
|---|---|---|
| Water Heaters are 4500W | 15,3555 | 61,419 |
| Each panel will receive 1,030 BTU/H under ideal conditions; 8 panels per array 8,000 | 32,000 | |
| Daily Savings, kWh | 9.4 | |
| Daily Savings / array @ $0.49/kWh | $4.60 | |
| Daily Savings, 11 arrays Total @ $0.49/kWh | $50.55 | |
| Maximum Potential Annual Savings | $18,450.28 | |
| Estimated ROI, Years | 1.5 | |
Cost to heat water:
- Full tank: 50 gallons (189 liters), from 86°F (30°C)4) to 122°F (50°C) = 4.4kWh or 15,013 BTUH
- Heating a full tank from cold:
- Electric Heat: 0.98 hours @ 4.5 kW/hr = $2.16 @ $0.49/kWh
- 4.5kW = 51 gallons per hour from 86°F (30°C) to 122°F (50°C)
- Solar Heat: 1.8 hours @ 8000 BTU/H (2.34 kW/hr) = $0.00
- 2.34kW = 26 gallons per hour from 86°F (30°C) to 122°F (50°C)
- Maximum temperature of 50 gallon tank in 4 hours of full sun: 163°F (73°C) (assuming no water use)
