A solar hot tub looks like a typical hot tub but uses the power of the sun in maintaining the desired water temperatures. A circulation pump extracts water from the tub and diverts the flow through specially designed solar thermal collectors. The panels conduct heat from the sun’s rays and transfer the free energy to the water. Once heated, the water returns to the spa. Solar hot tub pumps circulate water using conventional electricity or electric solar panels.
Solar hot tub heater panels have either a flat plate or an evacuated tube design. Flat-plate thermal panels consist of a metal box and a glass top. Within the box lies black tubing, no larger than about 0.4 inches (1 cm) in diameter, which conducts the sun’s rays and passes the heat on to the water. Evacuated tube panels contain the water pipe vacuum-sealed tube within one or two other glass tubes. The outer tube contains glycol, which retains and transfers heat to the water tube.
Evacuated tube designs are also contained within a glass-topped framed structure. The extra insulating value gives the system greater efficiency, because less heat escapes back into the environment. Homeowners typically install hot tub water heater panels on the south of the house that is closest to the equator, where they will receive the most sunlight. The panels can be mounted, such as on a roof, angled racks or a fence, or they can be placed on the ground.
Manufacturers construct thermal water panels, designed for sun-powered hot tub heating, in many sizes. A typical 80-square-foot (7.43-square-meter) panel provides as much as 80,000 British Thermal Units (BTU) of heating power. One panel sufficiently heats a 7,500-gallon (28,390-liter) solar hot tub. During peak sun hours, a solar thermal panel increases water temperature by as much as 10 degrees Fahrenheit (12 degrees Celsius).
The most cost-effective method of heating a solar hot tub involves a thermosyphoning system. Gravity, conduction and convection circulate water through the thermal panel, which heats and returns water to the spa without requiring mechanical pumps. Some systems use solar photovoltaic panels to supply electricity for direct current (DC) or alternating current (AC) pumps. One 68-watt solar panel generates enough power to operate a 12-volt DC pump. A one-horsepower AC pump requires about 115 volts of electricity.
Although DC pumps circulate only 3-5 gallons (11.35-18.9 liters) of water per minute, water remains in the thermal panel longer, thus acquiring a higher temperature. By contrast, a solar hot tub equipped with an AC pump moves water at the rate of 60 gallons (2,271 liters) per minute. Water passes through the thermal panel faster, but it increases in temperature by only a slight fraction and must be continually circulated.