Operation and Control

SWHS operation strategies

How to operate a SWHS depends a lot on its type:

Thermosiphon SWHS are completely operated by nature. The water starts to circulate in the hydraulic collector circuit, if the collector temperature is increasing due to solar irradiation and hence the water density is decreasing. As higher the temperature difference between the warmer water in the collector and the colder water in the above installed storage tank is, as faster is the water circulating.

Manual operation of forced circulation SWHS is sometimes the case, but rather an exception, because a reliable operator, stable weather conditions and constant hot water demand is needed, to manually operate a SWHS efficiently. If the irradiation intensity is changing fast due to fast changing weather conditions, a manual operator cannot follow and optimise the operation.


Automatic control of forced circulation SWHS is the standard operation concept in most countries, especially for large SWHS. The simplest version is a differential temperature control: The temperature at the collector outlet and at the bottom of the storage tank is measured and compared. If it is above a threshold, the controller starts the pump. If the water is circulating, the collector temperature is lower, since heat is removed. To avoid an immediate stop of the circulation due to the decreased collector outlet temperature, a hysteresis function must be implemented in the controller. This means, that the switch-on threshold temperature is higher than the switch-off threshold temperature (e.g. switch-on above 8 C and switch-off below 3 C temperature difference).

The right positioning of the temperature sensors is very important to assure an efficient operation. The collector outlet temperature sensor should be installed as close as possible to the outlet of the collector or should be fixed on the back of the absorber sheet. The storage bottom temperature (temperature of the water, which is pumped from the storage to the collector) must be measured at the same height than the outlet of the storage tank. If the temperature sensor does not extend into the storage tank, but is installed in the connection pipe, it must mounted very close to the storage tank, because the temperature in the pipe could be lower than in the tank.





If additional pumps are installed in the SWHS, e.g. to pump the water from the storage to the process, a second controller must be installed. Also in this case typically two temperatures are measured and the pump is switched-on and switched-off depending on the temperature difference. If an external heat exchanger is installed, pumps are needed on both sides of the heat exchangers, which can be controlled together. It must be assured, that the fluid flow on both sides is similar, to achieve an efficient heat transfer.

SWHS control checks

With the following checks the operator can control, if the SWHS operates correctly. However, the checks are not replacing a monitoring system.

The collector temperature should reach on sunny days between 60C and 90C depending on the efficiency of the collector and the water flow in the collector circuit. The temperature should be significantly higher, if the system is not operating (pump switched-off). But what counts is the temperature during operation.

The storage temperature at the top should be close to the collector temperature level, if the hot water from the collector field is fed in the tank at the top. If the water is mixed in the storage tank, the temperature should continuously increase during a sunny day and should approach the collector outlet temperature in the afternoon.

The storage temperature at the bottom should be as low as possible, at least in the morning, when the SWHS starts to operate, because the collector efficiency is higher at a low inlet temperature. The storage temperature at the bottom should be close to the cold water temperature of the process water. If the water in the storage tank is heated up during the day and a good stratification of the temperature levels is given, the temperature at the bottom will increase only if the storage is entirely heated up. If there is only a minor stratification, the temperature at the bottom will be heated up continuously with the entire water volume of the storage tank.

The water in the collector circuit should circulate when the sun is shining and the collector temperature is higher than the storage temperature at the bottom. This can be checked indirectly by checking, if the pump is switched-on. If a sight glass is installed in the hydraulic circle, the flow of the water can be checked directly.

The heat transferred to the process is, what finally counts, because it is saving fuels and costs. The heat is proportional to the temperature difference between the water delivered to the process and either the cold water entering the system (if the process water is flowing through the solar system) or the return flow from the process (if there is a hydraulic circuit between the solar storage tank and the process). The higher this temperature difference is, the more energy is delivered. But the heat transferred is also proportional to the mass flow, this means, if a large water volume is delivered to the process, the temperature difference is lower for the same amount of heat delivered.