Living off-grid: Hot Water

PUBLISHED 19 DEC 2016   

Once you have your rainwater harvesting system setup to enable you to go off-grid with water, you may want to reduce your electricity bill by installing a solar hot water system too.

Hot Water Off-Grid

Hot water produced in a conventional electrical geyser uses an exorbitant amount of energy. In fact, it is typically the most energy-hungry consumer in a home. By contrast, solar powered hot water systems provide an efficient and affordable alternative, and once the unit has been installed, will provide a free source of hot water to your home as long as you receive sunshine. In a country such as Namibia, where sunshine is not in short supply, solar hot water systems are extremely efficient.
There are various types of solar hot water systems available, but the simplest, most effective, and cheapest option is a solar evacuated tube hot water system. A evacuated tube solar hot water system consists of a number of glass tubes — which can vary depending on the size of the hot water system required — which are attached to a hot water cylinder that stores the heated water and supplies it to the home as needed.
During the daytime, the water inside the glass tubes gets heated by the sun's energy, and as it heats up it begins to rise up the glass tubes, flowing into the holding tank, which is usually positioned above the tubes. Because hot water is less dense that cold water, the heated water rises to the top of the cylinder, while the denser cold water at the base of the tank gets displaced, forcing it to flow out and refill the tubes where it too is exposed to the sun's rays. This cycle is continually repeated, ensuring that there is always hot water in the holding tank as long as the sun is shining. When you turn on a hot water tap in your home, heated water is drawn from the upper portion of the tank. This is replaced by cold water, which enters via an inlet at the bottom of the tank, which in turn flows into the glass tubes where it lies, waiting its turn to be warmed by the free energy provided by the sun before being cycled back into the holding tank.
This method of water heating is extremely efficient and requires no electrical pumps to cycle the water or electricity to heat the water. However, some families do prefer to have an optional thermostat and backup electric element tied to grid power to ensure there is always hot  water available, even during periods of extended cloud cover.
Evacuated tube solar hot water kits are available in both low-pressure and high-pressure units, with the latter usually costing more than the former. The capacity of the units typically range in size from 100 litres to 300 litres, with the number of tubes in the system ranging from eight to thirty. The bigger the holding tank, the more hot water can be stored; the more glass tubes fitted, the faster water heats up within the tank.
While these solar hot water kits may be cost effective for homeowners that are planning or building a new home, it can be quite a financial outlay for homeowners who already have an electric geyser installed. The good news is that there are also evacuated tube and flat-plate photovoltaic conversion kits available that enable homeowners to convert their existing electric geyser to solar.