In South Africa, the most prolific method to supplying hot water is with a conventional geyser. Through DSM projects, Eskom has promoted alternatives to hot water heating by means of geysers, such as solar water heating and to lesser extent heat pumps. In the USA, many more types of water heaters are used compared to South Africa. Table 2.5.5 provides a summary of the different types of water heaters, their cost to purchase, their cost to run and their life expectancy [35].
Table 2.5.5: U.S. Life cycle costs of various types of water heaters [35].
It is evident from Table 2.5.5 that the conventional electric geyser is by far the most costly way to supply hot water. The solar water heater with backup electrical element is the most costly to purchase compared to the other hot water heating methods.
2.5.3.1 Conventional geyser
A conventional geyser consists of an insulated hot water cylinder and an electric heater element. As hot water is drawn from the cylinder, cold water enters it. This cold water is at a temperature close to soil temperature at which the water supply pipe is buried. Inlet water temperature will be discussed in the last section of this chapter. The geyser element can range in size from two to
53 five kW depending on the water capacity of the cylinder. The element is controlled by a hysteresis-type relay. The water in the cylinder is kept between a certain preset temperature range by this relay which switches the element on and off. This range is normally set between 55 and 65 degrees Celsius. A study conducted by Bosman et al. evaluated the electrical saving achieved by installing additional insulation around the cylinder of a geyser. An average annual saving of approximately 18 percent was realised [36]. The standing losses that were quoted in the study were used in the modelling of the geyser in Chapter three.
2.5.3.2 Demand or instantaneous water heaters
Demand or instantaneous water heaters do not heat and store hot water like a geyser does. When the faucet is turned on the electric element heats the water directly, thereby eliminating the standing losses associated with the conventional geyser. The one drawback of these water heaters is that they have a limited flow rate of between 7 to 15 litres per minute [35].
2.5.3.3 Solar water heaters
A solar water heater captures the solar radiation which is converted to thermal energy which in turn heats up the water stored in an insulated cylinder. In the past, solar water heaters have not been a financially feasible option because of their high capital costs, relatively poor performance and low electricity tariffs. This situation has changed. Eskom has a DSM program in place that subsidises a part of the capital cost of the heater. Electricity tariffs have increased substantially over the past couple of years and this has resulted in a substantial reduction in pay-back periods for solar water heaters. The majority of the solar water heaters in South Africa are equipped with an electrical element and function similar to a conventional geyser in the way that the electric element is off when the water is within the preset range. In this way, hot water is always available provided there is a supply of electricity even if there has been poor solar radiation during a day. Current solar thermal systems can utilise 30 to 60 percent of the solar radiation that falls on their collectors. As a result of this, solar water heaters have improved substantially in the past few years and now provide a low maintenance alternative to a conventional geyser. There are three main types of solar water heaters namely: vacuum tube, evacuated tube and flat plate collectors. The tube type is able to passively track the sun whereas the flat plate type is sensitive to sun position [37]. Figure 2.5.7 depicts an evacuated tube type solar water heater.
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Figure 2.5.7: Evacuated tube collector type of solar water heater [38].
2.5.3.4 Heat pumps
Heat pumps work on the same principle as an air conditioning unit but just in reverse. The heat energy is transferred from a low temperature heat source to a higher temperature heat sink. The heat pump does not generate the heat directly; instead electricity is used to drive the compressor. Figure 2.5.8 shows the flow diagram of an air-type hot water heat pump.
55 For an equivalent amount of energy, a heat pump water heater can heat two to three times more water than a conventional geyser [35]. They can also offer the added benefit of space cooling because the heat energy is extracted from a living space and transferred to the hot water cylinder. Many of these types of heaters have a backup electric element to heat the water when the air temperature is too low for efficient operation. The installation costs of these heaters are higher than that of a geyser but the running costs are considerably lower. Geothermal water heat pumps can operate in colder climates than the previously mentioned air heat pump water heaters. They extract heat energy from the ground rather than the air which is colder in winter than the ground. The geothermal heat pumps are more costly to install than the air heat pumps water heaters [35].