Solar thermal domestic water heating systems

The heating of domestic water involves much higher temperatures than for swimming pool water. The simple absorbers used for swimming pool heating are in most locations not suited to domestic hot water systems because the absorber losses due to convection, rain and snow as well as heat radiation are unacceptably high. Domestic water heating systems typically use collectors that have much lower losses at higher water temperatures. These are either flat-plate, evacuated flat-plate or evacuated tube collectors and are integrated with collector storage systems. Collectors for domestic water heating are described in the section on solar collectors, p85.

A complete system for domestic water heating consists not only of a closed collector to heat the water. Further components such as a hot water storage tank, pump and an intelligent control unit are needed to ensure a hot water supply that is as comfortable as we expect from conventional systems.

A very simple system for solar water heating can be made of a black water-filled tank that is exposed to sunlight in summer. If the tank is installed higher than the tap, the warm water can be used without any further component. An example for such an application is a solar shower that is sold as camping equipment. In principle, it is a black sack hung on a high branch of a tree. If this sack is exposed for some hours to solar radiation, a shower with solar heated water can be taken.

However, this system does not meet the demand of daily routine. After the sack is empty it must be refilled again by hand. To avoid this inconvenience, sack and tap can be pressure-sealed and a hose can then be connected to replace water automatically. As a further improvement a solar collector with a high efficiency all year round can replace the sack. However, the collector content is only sufficient for a very short shower and the water temperatures will be very high. Therefore, a storage tank is needed. Two systems to integrate hot water storage tanks into solar energy systems are described in the following sections.

Thermosyphon systems

A thermosyphon system as shown in Figure 3.3 makes use of gravity. Cold water has a higher specific density than warm water. It is therefore heavier and sinks to the bottom. The collector is always mounted below the water storage tank. Cold water from the bottom of the storage tank flows to the solar collector through a descending water pipe. When the collector heats up the water, the water rises again and flows back to the tank through an ascending water pipe at the upper end of the collector. The cycle of tank, water pipes and collector heats up the water until temperature equilibrium is reached. The consumer can draw off hot water from the top of the tank. Used water is replaced through a fresh supply of cold water through an inlet at the bottom

Hot water

Hot water

Thermosiphon Flat Plate Schematic
Figure 3.3 Schematic of a Thermosyphon System

of the tank. This cold water joins the cycle and is heated in the collector in the same way as before. Due to higher water temperature differences at higher solar irradiances, the warm water rises faster than at lower irradiances and the flow rates are increased. Therefore, the water circulation adapts itself nearly perfectly to the available solar irradiance.

It is very important that the storage tank of a thermosyphon system is well above the collector; otherwise the cycle can run backwards at night and cool the water from the storage tank over the collector. In regions with high solar irradiation and flat-roof architecture, storage tanks are usually put on the roof. The collector is also mounted on the roof or on the wall of the sunny side of the house.

With gable roofs, the storage tank must be mounted as high as possible under the roof if the collector is also installed on the roof. The high weight of the water-filled tank can sometimes cause structural problems. Furthermore, integration with a conventional heating system, which is usually placed in the basement, is more difficult.

A system where the water flows directly through the collector is called a single-circuit system. Such a system is only suitable for frost-free regions; otherwise the water can freeze in the collector and pipes and destroy the system. In regions with the possibility of frost, a double-circuit system is frequently used, in which the water is kept inside the storage tank. A second quantity of water is mixed with an antifreeze agent to use as a working fluid in the solar cycle. A heat exchanger transfers the heat from the solar cycle to the storage tank, thus separating the usable water from the antifreeze mixture. Glycols are often used as antifreeze agents; however, antifreeze agents should be non-toxic because they can contaminate the hot water supply in the case of a system failure. Therefore, ethylene glycol, which is used for many technical applications, is not used for solar energy systems. To avoid corrosion damage, the antifreeze agent must also be compatible with the materials used in the system construction.

Thermosyphon systems also have some important disadvantages. The system itself is inert and cannot react to fast changes in the solar irradiance. Thermosyphon systems are usually not suitable for large systems with more than 10 m2 of collector surface. Furthermore, the storage tank must always be installed higher than the collector, which is not always easy to realize. The collector efficiency can also decrease due to high temperatures in the solar cycle. However, thermosyphon systems are very economical domestic water heating systems. The principle is simple and needs neither a pump nor a control system. Therefore, the system cannot fail due to a fault in these components. Finally, the energy to drive the pump and control system is saved.

Systems with forced circulation

In contrast to thermosyphon systems, systems with forced circulation use an electrical pump to move the water in the solar cycle. The collector and storage tank can be installed independently and a height difference between the tank and collector is no longer necessary. However, the pipe lengths should be designed to be as short as possible since all warm water pipes cause heat losses. Figure 3.4 shows a system with forced circulation.

Two temperature sensors monitor the temperatures in the solar collector and the storage tank. If the collector temperature is above the tank temperature by a certain threshold, the control starts the pump. The pump moves the heat transfer fluid in the solar cycle. The switch-on temperature difference is normally between 5 and 10°C. If the temperature difference decreases below a second threshold, the control switches the pump off again. The choice of both thresholds must ensure that the pump does not continually switch on and off during low irradiance conditions.

Conventional circulation pumps made for heating installations can be used for the solar cycle. These pumps are reliable and economic. Most pumps have various velocity stages to adapt the flow rate to the solar irradiance. Pumps are usually designed for flow rates of 30-50 litre/h per square metre of solar collector area. Higher flow rates are chosen for swimming pool absorbers since the temperature requirement is lower and the water needs less heating. However, if the flow rate is too low, the temperature in the collector rises and the system efficiency decreases. On the other hand, if the flow rate is too high, the energy demand to drive the pump is unnecessarily high.

The pump usually runs at the alternating voltage of the public grid. It is also possible to use DC motors to drive the pump. A small photovoltaic system can provide the electrical energy needed. In that case, all of the energy for the system comes from the sun.

Water Pump Free Energy Device
Figure 3.4 Schematic of a Double-Cycle System with Forced Circulation
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