Thermosiphon Solar Water Heating

By Kevin Wei on 03/06/2012

Solar energy is clean, ubiquitous and inexorable. Solar thermal systems are the low-hanging fruits compared with photovoltaic systems. What’s more, it is highly desirable to use solar energy to reduce or avoid the escalating costs of gas, oil or electricity, most of which is generated by coal-fired power plants. The cost of installing solar heating systems can often be recouped by utility savings within a few years, especially since federal and state governments offer strong financial incentives through tax credits. However, most homeowners or businesses have not gone solar yet. This may be due to the relatively high upfront costs of installing equipment compared with traditional heating systems (even though the total equipment and operating costs of traditional heating systems are far higher in the long run).

A solar water heater commonly consists of a solar collector; a controller and thermostat connected with temperature sensors installed at the solar collector and at the solar water tank; a pump assemblage; and a solar water tank with an internal heat exchanger. Such a system is generally difficult for most homeowners to comfortably and reliably install. Generally, the process requires two professional installers two or more days to set up the system, at a total cost of $5,000 to $7,000.

Furthermore, there are some vulnerable parts to the otherwise reliable solar heater. Controller electronics can be damaged by spikes in power supply or lightning, sensors and thermostats can malfunction because of bad contacts, and a pump can fail to start or burn out. When utility power is out, a traditional solar water heater will not work and may even be damaged. These problems usually account for 90 percent of service calls.

However, there is a type of solar water heater that uses an evacuated-tube solar collector and eliminates the expensive and vulnerable hardware, reducing the equipment cost by half, and making installation simpler and the solar water heater much more reliable and durable (see Fig. 1). Many homeowners can even install the system themselves.

The specs are complicated, but worth knowing. An evacuated-tube solar collector is plumbed directly to a conventional electric water heater tank. The top of the solar collector should be positioned below the bottom of the tank. This means that the solar collector can be installed on the ground, on a slope below a building on the exterior walls, as window awnings or on a lower-level roof. The water tank costs less than half the price of a solar water heater tank with an internal heat exchanger. When the sun comes out, it heats the water in the solar collector. The heated water in the solar collector will rise, just as warmer air or any fluid will rise, because it is less dense. The warmer water will enter the water tank at the pressure- and temperature-relief valve level and rise to the top of the storage tank. At the same time, cooler water from the bottom of the tanks is drawn down to the solar collector through the pipe that connects to the bottom of the storage tank at the left end of the solar collector (see Fig. 1). This end has a “U” shaped pipe that prevents warmer water from rising from this end of the collector and entering the bottom of the storage tank. The clockwise water circulation in Fig. 1 continues as long as there is sunshine, gradually heating up the water in the tank, resulting in increasingly warmer or increasingly hotter water in the tank. This natural circulation is called thermosiphon. It requires no pump, and is the most reliable and durable operation.

Here’s another option: A water heater can be installed or an existing water heater can be plumbed to the tank described above. This will increase water-storage capacity, and is a preferred setup especially for a retrofit project where there is already a water heater in place. In this case, the higher positioned tank that is plumbed directly to the solar collector does not need an active back-up heat source, such as gas or electric. In terms of efficiency, the double tank system is only slightly better; there is also trade off because there are two tanks from which to lose heat. The single tank system does not store as much hot water as the double tank system, but the evacuated tube collector can heat the water significantly higher and thus can supply more hot water than a normal water heater.

Thermosiphon solar water heaters with flat plate collectors have been widely used in non-freezing climate areas, but they are not used in freezing climate regions: Flat-plate collectors don’t have thermal insulation for the manifolds and riser tubes, which can freeze and break. The thermosiphon solar water heater with evacuated tube solar collector described above overcomes the freezing problem because the evacuated tube collector and the pipes have much better thermal insulation than pipes in unconditioned space in most homes. A double safety mechanism is added by installing a freeze-prevention valve, a relatively inexpensive, non-electric, thermally controlled valve. It contains a precise thermal actuator that opens the valve slightly to drip water when the temperature of the pipe approaches 350 degrees (with outside temperature much lower) and closes the valve when warmer water flows to replace the near freezing water.

For places where it is not convenient or possible to install the solar collector below the water tank, natural thermosiphon circulation cannot be used. Otherwise, a PV-powered pump connected to a PV panel can be installed to provide forced circulation. This adds an extra cost of about $300 compared with the thermosiphon-only water heater. When antifreeze is preferred to circulate through the solar collector and a heat exchanger is used to pass the heat from the heated antifreeze fluid to the water, a PV-powered pump connected to a PV panel is also highly recommended as this design is simpler, less costly and more reliable then the sensor-thermostat-controlled system design. Once there is hot water, it is relatively simple to circulate hot water to radiators, incorporate into existing forced air system to blow the heat through out a building or heat a building through radiant floor heating.

Kevin Wei, owner of SolarPlusGreen.com, holds a N.C. General Contractor/Home Builder License, National Association of Realtors’ Green Designation, EarthLinked geothermal heat pump installer certificate and a North American Board of Certified Energy Practitioners’ PV installer certificate.