Aloha! I want to build a solar-powered hot tub, and wonder how to keep the water at a constant temperature of about 103°F? Years ago, I made my own domestic solar hot water system. It worked well, but there was a wide variation in temperature depending on the sun conditions and the level of hot water usage.
Karim Wingedheart • via email
I assume from your greeting that you are from Hawaii. The combination of plentiful sunshine and the highest energy costs in the United States would make investing in a solar water heating (SWH) system for your hot tub a smart idea.
But don’t expect the SWH system to do it all. In HP104, Floridian Bob Owens describes his solar-heated hot tub, which needs help from the electric heater at times. Hot tubs are known to lose heat quickly when the jets are on, which is why many tubs are heated with a hardwired 240-volt, 50-amp dedicated circuit. (A 120 VAC resistance tub heater with a 20 A circuit has only 25% of the heating capability of the 240 VAC heater.)
The Owens’ single-collector SWH system heats his 360-gallon hot tub to 104°F just as an AC heater does, but it won’t keep up with the heat loss when the jets are on—losing 0.5°F to 1°F per minute. A 360-gallon tub losing 1°F per minute needs about 3,000 Btu of heat added per minute to overcome this loss and keep the tub temperature above 100°F. The heat loss will vary with the ambient temperature.
Any solar energy system capable of meeting this intermittent load would be so large that its size and cost would be prohibitive. The only solution is storage. In the case of a SWH system similar to the one described in HP104, a larger storage tank and more collector surface area would be required. A possible alternative is a grid-tied PV system—with the grid “supplying” the “storage.” In any case, the heat loss, size of the tub, and amount of time used per day are needed to know how to design an effective system.
If we stick with the 360-gallon hot tub and assume a loss of 1°F a minute for an average of 30 minutes a day, we have a daily loss of 90,000 Btu per day, or an electrical equivalent of 26.4 kWh per day. In Hawaii, an SWH system would require about three 4- by 8-foot collectors and a 120-gallon tank to keep up with the load. The tank water would be heated daily to 140°F to 160°F. To keep up with the heat loss, a pump of about 10 to 15 gpm (a medium-head pump) would be needed to transfer the heat from the storage tank to the tub.
In Hawaii, a batteryless grid-tied PV system that could supply the 26.4 kWh per day would need to be about 6 kW (DC). At an installed cost of $5 per watt, this system would cost approximately $30,000 (prior to incentives). The SWH system would cost about half that. This would change depending on applicable tax incentives and does not address the significantly reduced energy required to maintain the hot tub temperature with its cover on.
Please keep in mind this is a hypothetical scenario. Changes in heat loss, usage per day, installation location, and time of the year will all affect system size and performance. For example, an outdoor tub installation in Phoenix on a summer day won’t have any heat loss, while the same tub in November in Seattle will.
Chuck Marken • Home Power solar heating editor