Domestic hot water (DHW) is water intended for human consumption (potable) that has been heated. It is used for sanitary uses (bathrooms, showers, etc.) and for other cleaning uses (washing dishes, washing machine, dishwasher, floor scrubbing). In terms of energy, the DHW is an important component to take into account, since it represents between 25 and 40% of the energy consumption of homes.
As a general rule, it consists of: the water heater, circulation pump, pipes, flow meters, valves for the distribution of hot water to consumers or points of consumption.
Two types of facilities can be distinguished:
- Hot water open circuit: in case the water from the same heating circuit is the one used in domestic hot water applications.
- Hot water closed circuit: in case hot water is recirculated thanks to a pump in an independent circuit, so during its journey through the circuit a heat exchanger or a domestic hot water tank is interposed transmit the heat produced in the main hot water circuit.
Use of solar thermal energy to obtain sanitary hot water
One of the possible ways to obtain sanitary hot water is using the Sun by means of a solar thermal installation prepared for this purpose. The solar collectors are responsible for capturing solar radiation and transmitting this energy to a fluid. The fluid increases its internal energy and its temperature. This fluid circulates through a thermal interchange that allows to transmit thermal energy to the water that we want to heat. The hot water can be transported, therefore, it can transport the energy. The heated water can be stored in a well thermally insulated tank or circulated through a heating circuit to heat a house.
Obtaining domestic hot water is one of the most widely used applications of solar thermal energy. This application is also called "hand water".
Aspects of solar energy to be considered in a solar hot water installation
The solar radiation received in a place is determined by the angle formed by the sun's rays with respect to the ground. In summer the sun is more perpendicular to the ground than in winter, so in summer we receive more solar radiation.
The energy needed to heat domestic hot water up to a certain temperature is determined, among other things, by the temperature of the water before heating it. Therefore, in winter, the water that comes from the network in general is colder, more thermal energy will be needed than in summer.
The solar hot water systems require a water storage tank so that the water that is heated is stored in a well-insulated tank for when it is going to be used. This is essentially due to two reasons: on the one hand the difference in solar radiation received during the different hours of the day, not to mention the null solar radiation that is received during the night and, on the other hand, because we are not going to use the hot water constantly, that is, most of the day we will not need water but when we wanted to shower we would need to heat a lot of water in a very short time.
Economically you can not pretend to size a solar thermal installation to obtain all the sanitary hot water that we need throughout the year. In this case, we would have to oversize it a lot in order to obtain a large amount of heat energy in winter. This energy would be left over in summer, which would cause the water in the storage tank to boil, generate water vapor and the pressure of this steam would end up bursting the tank.
One solution would be to throw hot water and add cold water but it would not be reasonable either economically or environmentally.
There are many regulations in different countries in which it is required that solar thermal installations for obtaining domestic hot water are sized for a reasonable consumption in summer and have an auxiliary system of reheating when the system can not reach the temperature of the desired water.
Solar hot water installations
There are two types of sanitary hot water installations (or heaters):
- Open circuit sanitary hot water installations. The drinking water passes directly through the solar collectors. This system reduces costs and is more efficient (energetically speaking), but presents problems in areas with temperatures below the freezing point of water, as well as in areas with a high concentration of salts that end up clogging the conduits of the panels.
- Closed circuit sanitary hot water installations. Two systems are distinguished: thermosyphon flow and forced flow.
Solar thermal panels have a very low environmental impact.
Characteristics of water The most important element of this type of solar thermal installations is water. Due to the characteristics of operation and use of these systems, it must be ensured that the water is within certain restrictions and temperature ranges.
Domestic hot water is subject to different thermal restrictions, which regulate the distribution and consumption temperatures, and which increase the complexity of the projection of the domestic hot water installation. The minimum temperature of the installation is restricted, to avoid the appearance of legionella.
In summary, the reduction in temperature to less than 50 ° C in the tap and 55 ° C in the tank should be avoided under normal operating conditions. The installation as a whole should be able to withstand temperatures of 70 ° C.
On the other hand, it should be avoided to contact water at temperatures above 40 ° C with the skin, to avoid possible burns. This can be achieved by mixing the hot water from the DHW installation with conventional cold water, using mixers.