Methods for heating water
Several methods are used for heating water, either directly or indirectly. The latter case requires avail-able sources of hot water that can be used to meet all or a part of the heating requirements. When using direct methods, all necessary energy must be added to the water. Electricity, oil or gas are the usual sources of energy for direct heating of water.
It is better, however, if other available energy sources can be used to meet part or all of the energy requirement. These can be separated into low and high temperature sources. High temperature sources can be used to meet the entire need for heating, if the amounts are large enough as they are much hotter than the required final temperature. Hot industrial effluent water or geothermal water are examples of such sources.
Low temperature sources are hotter than the raw water to be heated but their temperature is insuffi-cient to meet total energy needs. The source can therefore be used as a part supply to the total water heating requirements. Seawater or groundwater are examples of sources that can be used to meet part of the energy needs; industrial effluent water can also be included in this category.
Whether industrial effluent water or geothermal water can be used directly for the fish, or whether heat exchange is necessary depends on its quality. Normally it is necessary to use a heat exchanger, because the quality of industrial effluent water is not continuously good enough for direct use as growing water for the fish. Low temperature sources are also suitable for use with heat pumps .
When supplying energy to heat water, it must be utilized as efficiently as possible. Recovery of energy from the outlet water is therefore common, either by direct or indirect systems. Re-use systems are direct, with the water being used again directly, while heat exchangers where only the energy is re-used, represent indirect systems. It is not possible to recover all the energy that is added by using heat exchangers. Even if it were theoretically possible, it is not cost effective because the heat exchanger would need to be very large and expensive.