Domestic hot water is typically one of the largest energy loads in a residential building. Solar water heating — using the sun to heat water for bathing, cooking, and washing — can supply 60 to 80 percent of domestic hot water needs in most climates, dramatically reducing the electrical load on the off-grid energy system.

Passive solar water heating systems (also called thermosiphon systems) use natural convection to circulate water between a collector panel and a storage tank without any pump: cool water at the bottom of the tank flows to the collector, heats up, becomes less dense, and rises to the top of the tank. These systems have no moving parts, require no electricity, and are highly reliable. They require that the storage tank be located above the collector panel, which is usually accomplished by mounting the tank on the roof adjacent to the collector.

Active solar water heating systems use a pump to circulate water or a heat transfer fluid between the collector and the tank. They are more flexible in terms of equipment placement and can achieve higher performance than passive systems, but they require a small amount of electrical energy to run the pump (typically met by a small dedicated PV panel). Active systems are also more complex and require more maintenance than passive thermosiphon systems.

Review Questions

1. Write out the formula for calculating annual rainwater collection from a roof. What factors affect collection efficiency, and what typical values would you use for a metal roof?

2. What is a first-flush diverter, and why is it an important component of a rainwater harvesting system? How much water should a first-flush diverter capture per square meter of roof?

3. Describe three reasons why integrating a cistern into the interior of a building might be preferable to placing it underground outside the building.

4. List the stages of a complete water filtration system from cistern to potable tap. What contaminant or contaminants does each stage address?

5. Compare passive (thermosiphon) and active solar water heating systems. What are the advantages and disadvantages of each? Under what circumstances might you choose one over the other?