5 Passive Solar Water Heating
A passive solar water heating system uses natural convection or household water pressure to circulate water through a solar collector to a storage tank or to the point of use. Active systems employ pumps and controllers to regulate and circulate water. Although passive system are generally less efficient than active systems, the passive approach is simple and economical.
Passive water heating systems must follow the same parameters for installations as active systems - south facing unshaded location with the collector tilted at the angle of our latitude. Since the storage tank and collector are combined or in very close proximity, roof structural capacities must accommodate the extra weight of a passive system which can be 300 pounds or more.
5.1 There are two types of passive water heaters: batch and thermosiphon
5.1.1 Batch System
The batch system is the simplest of all solar water heating systems.
Schematic for Ground-Mount Batch Domestic Water System
It consists of one or more metal water tanks painted with a heat absorbing black coating and placed in an insulating box or container with a glass or plastic cover that admits sunlight to strike the tank directly. The batch system's storage tank is the collector as well. These systems will use the existing house pressure to move water through the system. Each time a hot water tap is opened, heated water from the batch system tank is removed and replaced by incoming cold water.The piping that connects to and from the batch heater needs to be highly insulated. On a cold night when no one is drawing hot water, the water in the pipes is standing still and vulnerable to freezing. In many applications, insulated polybutylene piping is used because the pipe can expand if frozen. The water in the batch heater itself will not freeze because there is adequate mass to keep it from freezing.
Since the tank that is storing the heated water is sitting outside, there will be heat loss from the tank during the night. This can be minimized by an insulating cover placed on the heater in the evening.
The most effective use of a batch water heater is to use hot water predominantly in the afternoon and evenings when the temperature in the tank will be highest.
Manufactured batch heaters have a "selective surface" coating on the tanks that will absorb heat most readily yet permits very little heat loss. This feature is very valuable in these type of systems as it helps insulate the tank.
5.1.2 Thermosiphon Systems
Thermosiphon System
The thermosiphon system uses a flat plate collector and a separate storage tank that must be located higher than the collector. The collector is similar to those used in active systems.
The storage tank, located above the collector receives heated water coming from the top of the collector into the top of the storage tank. Colder water from the bottom of the storage tank will be drawn into the lower entry of the solar collector to replace the heated water that was thermosiphoned upward. The storage tank may or may not use a heat exchanger. The thermosiphon system is more costly and complex than the batch system. In our area, it is best to use an indirect system (one that employs a heat exchanger). In that case, antifreeze can be used in the system eliminating freeze ups.
5.1.3 Sizing
The sizing of a batch system and thermosiphon system are both based on a usage figure of 20 gallons of hot water per person per day. For example, if the storage tank in these systems is 40 gallons, that would equal the requirement for two people. The collector area in the thermosiphon system should equal approximately 20 square feet per person.
The system is not sized for 100% of the energy requirement. A backup source is needed.