Stories from Casa Isabel

Solar Hot Water

1. Select a location with at least 5 hours of direct sun, preferably between 11am and 4pm.

2. Build a sloping framework to support the weight of cement, rigid insulation and water-filled 1″ coiled black PVC hose. We used steel C-channel, well sealed and painted, to support 2″ of rigid foam insulation with a skim coat of cement (painted white) on the top, and a coat of cement reinforced with rebar on the bottom.

3. Many variables determine the ideal angle of the sloping framework. Smaller units may be designed for seasonal adjustment, as the ideal is 90 degrees to the angle of the sun. Non-adjustable units should be angled to optimise your use cycle. For instance, our panels are at 90 degrees in the winter months, when shorter sun exposure makes solar gain more difficult. The off-square orientation is less important in the spring and summer, and in fact helps avoid overheating the system in the hotter, sunnier seasons. Solar panels in locations with icy winters will need to be pumped dry before they freeze, and will be better oriented to spring/fall sun angles. The use of glycol in such systems is beyond the scope of this homegrown eco-engineer.

4. Lash coils of 1″ black PVC hose together into stable circlular coils. Non-third-
world-country dwellers may find plastic spacers that the pipe clips into. (It is not tons of fun to discipline a roll of hose into a flat circular coil.) Our two panels use 200 meters of PVC in a 3×3 meter square (about 10´x10´) and 300 meters of PVC in a 4×4 meter square (about 12´x12´).

5. Cover panels with UV resistant woven greenhouse plastic and secure the edges.

6. The solar panels are pumped 3 or 4 times per day into super-insulated tanks. We started out using the locally common and economical Rotoplas 1100 litre (about 300 US gallons) plastic tanks, super-insulated with sawdust and rigid insulation. We have since moved to thicker, more durable white plastic cisterns of the same size, to overcome leaks due to differential expansion of plastic and metal. Big budget people can use steel tanks. Ideally the tanks are close to the panels, but we have tested a system where the tank is 100 meters (350 feet) from the panel and it is about 25% less efficient due to line loss.

7. Electric 3/4¨pumps connected to the bottom of the tanks pump the hot water out of the panel and into the top of the tanks. Each panel has its own pump and separate tank, as well as a digital timer or digital temperature controller (sensor).

8. We use a 3 tank system, with all tanks at the same level. The first tank is not connected to the panels, but instead is a fill tank with a float valve, to maintain the level in all 3 tanks when the hot water is used in the houses.

9. Tank one (the fill tank) fills tank two (the pre-heating tank) after passing through the larger panel. The heated water off the top of tank two enters the bottom of tank three, the use tank. This design avoids the problem of ever adding cold to the (hottest) use tank and allows a 24/7 duty cycle.

10. Timers are often sufficient to accumulate enough thermal momentum, but cloudy afternoons can lower efficiency. Depending on your weather and system demands, temperature controllers with sensors, though more difficult to install, raise the efficiency of the system because they only deliver water that is hotter than the temperature you select.

Are your eyes rolling around in your head yet?

11. Over the last 35 years Chris has built about a dozen different solar hot-water systems, starting with a simple static coil feeding off a 45(55US) gallon drum and going to a shower head. It cost about $50 and lasted 25 years. Along the way there were thermal-siphoning pool and house heating units and a coil inside a big old satellite dish which easily tipped to capture morning and evening sun. We tried 180 degree turns with steel; plastic hose on steel manifolds for high volume, low temperature gain; and various sizes of plastic tubes, eventually pinning down optimum designs. The system at Casa Isabel started with a single tank and a small thermal siphoning panel. As demand increased it could not keep up because thermal momentum was lost due to the inefficiency of thermal siphoning as well as adding cold to the use tank, thus cooling what was already hot. These problems were solved by using pumps, timers and temperature sensors.

12. For unknown reasons solar gain is almost non-existent here on cloudy days, quite different than in the far north where we started. For emergency backup during Yelapa´s rare cloudy spells, we have a 1000 watt temperature controlled backup heater in the use tank, which kicks in if the water in the use tank drops below 105 F(40 C).
We estimate that these panels, using 500 meters of 1″ black PVC hose in 5 hours of direct sun, produce more hot water than the 1000 watt heater running steadily for 24 hours. When fully charged the system has 2 tons of hot water on hand for 24/7 use, and provides showers for 30 people staying in houses on a two-acre hillside, readily accommodating the line loss to distant houses. So you turn on the shower and wait a minute or so, dee dee dee….
The thermal momentum is such that we once provided showers for 14 during a rare 5 day cloudy and rainy spell, when solar gain was negligible and the back-up heater did not need to come on. It is intriguing to consider that the water was heated in the sunny weather before the guests jumped on the plane.

12. The potential to use solar hot water to heat buildings, swimming pools or hot tubs is limited to the imagination, budget and potential space of the builder. Non-adjustable or fixed panels can only effectively harvest 5 or 6 hours of direct sun. Perhaps your requirements are not large and a tracking panel would be worth it. Proportion is the key, as systems can be designed to meet needs.

It seems like such a waste to see flat rooftops under a big sky in sunny climates not being used to capture solar energy.

Solar hot water has had a slow start as many designs have been tried and found less than ideal. Plastic fittings break and glass tubes explode, and more fittings means more potential for leaks. Gradually these obstacles are being overcome. Specific site and purpose design is the key to success in getting what you want.
Go people go!

© 2018 Casa Isabel

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