Increasing the efficiency of a district heating/cooling network using consumer performance information

RETOUR increases the efficiency of a district heating or cooling network by intelligently controlling the network supply temperatures, using consumer performance information. It is a solution that addresses the commonly known low delta T syndrome many existing networks suffer from.

Adjustment of the supply temperature is a common control philosophy to operate district networks. Most often it is done following the rule-of-thumb ‘weather compensation control’ based on outdoor temperature. Also, most of the in-house heating/cooling units are equipped with thermostatic control valves which have reference levels that refer to the heating/cooling rate. These valves can result in overheating/overcooling (or underheating/undercooling), especially when the supply temperature is increased/decreased by the network operator, respectively at DH/DC.

As the rule-of-thumb method does not guarantee optimized utilization of energy, the challenge was to develop a method that guarantees to have – at all times - the lowest (DH)/highest (DC) return temperature, in order to enable optimum energy usage.



VITO solution

RETOUR, a new control system, allows information to be fed accurately to the energy supplier about the return temperature at unit level. Self-driven control valves adjust the flow rate in the in-house pipeline automatically in order to achieve the set-point temperature that is defined by the consumer. When the flow rates of the in-house units reach an undesirable level (which means ‘high return temperature’ or ‘low return temperature’), the supply temperature at DH/DCis increased/decreasedrespectively. This action results in decreased/increased return temperature from the in-house unit at DH/DC, respectively.

How it works
Understanding the principle of RETOUR can be best explained with e.g. by considering an old high-temperature existing DH network. At the beginning of the heating season the supply temperature is typically set to 55°C. Colder periods will result in a higher return temperature from the consumers, due to the performance of the consumer unit i.e. radiator with thermostatic radiator valve demanding high mass flows. With the RETOUR control unit, a counteraction can be set in motion by intelligent increasing the supply temperature, for example to 60°C. Considering demand response by the consumers, the peak coldest period can still be supplied at a supply temperature lower than the design supply degree, because existing radiators are usually over-dimensioned as indicated in literature.

The rule-of-thumb of resetting the supply temperature according to outdoor temperature is a common control methodology that is considered in operating both the DH and the DC systems. The new RETOUR invention controls the supply temperature of DH/DC according to the performance of the in-house units. This avoids any inefficiency and/or over/under-heating at the consumer’s unit, which may happen when using the rule-of-thumb. Moreover, low supply temperature at the long-lasting off-peak heating periods provides significant savings at the heat production side.

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