One of the uses of solar thermal energy is obtaining sanitary hot water (DHW). Solar collectors capture energy from solar radiation to increase the temperature of a fluid.
Domestic hot water (DHW) is water intended for human consumption that has been heated. The primary uses are:
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Sanitary uses such as bathrooms, showers, etc.
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Cleaning uses such as dishwashing, washing machine, etc.
In energy terms, a domestic hot water system implies significant energy savings as it represents between 25 and 40% of the energy consumption of homes.
Domestically, water is traditionally heated in electric water heaters, gas-fired hot water generators. However, obtaining solar DHW is a way of heating water using renewable energies and being eco-friendly. In the case of heating systems, it is an alternative to the heat pump.
How does a domestic hot water system work?
The easiest way to obtain sanitary hot water is through solar energy.
Hot water can be transported; therefore, it allows to transport energy. The heated water can be stored in a well-insulated tank working as a storage water heater or circulated through a heating circuit to heat a home.
The essential elements that make up an ACS installation are:
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Water heater
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Circulation pump
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Pipelines
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Flowmeters
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Pressure relief valve
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Valves for the distribution of hot water to the points of consumption.
Obtaining sanitary hot water is one of the most used applications of solar thermal energy. It is a good option for single-family homes and also for neighborhood communities.
Performance of a solar domestic hot water installation
The angle determines solar performance that the sun's rays form with the ground. In summer, the Sun is more perpendicular to the ground than in winter, so that in summer, we receive more solar radiation.
The energy required to heat domestic hot water to a specific temperature depends on the water’s initial temperature before going through the circuit. Therefore, in winter, when the water from the network is generally colder, more thermal energy will be needed than in summer.
Solar sanitary hot water installations require a water accumulator to store the heated water in a well-insulated tank until we need to use it.
It is essentially due to two reasons:
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Due to the difference in solar radiation received during the different hours of the day, not to mention the zero solar radiation received at night and,
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Because we are not going to use hot water always, that is, most of the day we would not use water, but when we wanted to shower, we would use a lot of hot water in a short time.
How to size an ACS system?
From an economic point of view, it is impossible to pretend to size a solar thermal energy installation to obtain all the domestic hot water that we need throughout the year. In this case, we would have to plan a much larger structure to get a large amount of heat energy in winter.
This energy would be leftover in summer, which would cause the water in the storage tank to boil, generate steam, and the pressure of this steam would end up bursting the tank.
A solution would be to flush hot water and add cold water, but it would not be reasonable neither the cost nor the impact on the environment.
There are many regulations in different countries that regulate the size of solar thermal energy installations for ACS. The objective is that they are built for reasonable consumption in summer and that they have an auxiliary system when the system cannot reach the desired water temperature.
Types of DHW systems
There are two types of water heaters:
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Open-circuit sanitary hot water installations. The drinking water passes directly through the solar collectors. This system reduces costs and is more efficient because the heat loss is minimum. However, it presents problems in areas with shallow temperatures or with a high concentration of salts.
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Closed-circuit sanitary hot water systems work in a closed water loop and a heat exchanger. There are two different systems: thermosyphon flow and forced flow.
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Instant-systems heat the water on demand; when you need hot water, it flows through the collectors.
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Storage-systems are the most common system; hot water is stored in hot water tanks to supply when needed.