HOT | COOL NO. 4/2012 - Interface, Users and Utilities

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CONCEPT OF SOLAR AND PULSE HEAT In the period 2008-2011, 18 low-energy houses in the cooperative society in Hjortshøj have been built with solar connected to district heating and pulsating district heating (pulse heat). The pulse heat heats the heat storage tanks in each house. For the pipe network flexible PEX double pipe with 16 mm pipes from Starpipe and Logstor have been used. In this project following methods for reducing heat loss in the district heating supply have been used: - All homes are equipped with solar so the heat can be turned off in summer, typically four months a year. When the power for district heating supply is turned off, solar supplies most of the heat demand for hot water, with a small contribution of electric heating, as all houses have electric heating cartridges in their hot-water tanks. - There is a storage tank of about 500 litres in each house, which is heated, while there is district heating pulse, and which supplies heat and hot water to the house during breaks between the heat pulses. The tank also acts as heat storage for the solar heating system; it contains the district heating water. Utility water is produced in the most houses with a hot water heat exchanger connected to the tank. - In order to send water from the return pipe to flow at the end of a pulse, there is a pump at the joint district heating entrance to the housing groups, which can circulate water from return to flow when the plant's main valve is closed. This minimizes heat loss from flow in the pulse break.

In each house there are two thermostats, respectively at the bottom and middle of the tank. When both thermostats are above their set temperature, the tank is hot, and the pulse to the house is stopped. The following modes of operation exist: - Summer, i.e. June-September: the district heating supply is usually closed, but individual pulses can be manually started during periods without sun.

- Transitional period, i.e. October-November and April-May: daily pulse of 4-10 hours.

- Normal winter, i.e. December-March: two daily pulses of 4-6 hours.

- Particularly cold periods, renewable frost: permanent heating supply.

In the planning phase, it was estimated that this combination of modes of operation can halve the transmission loss in with the new buildings from about 25% of the heat supplied to about 12%. In parallel with the pulse operation a data acquisition system was installed for the houses’ heat meters as well as two joint meters. With this measuring system, heat data is collected on an hourly basis.

OPERATING RESULTS WITH PULSE HEAT The operation is carried out with the different modes of

operation specified above. However, the pumping action of return to flow has not been started because of problems in managing this. For the first heating year (October 2009-September 2010) the pipe loss for the one housing group amounted to 8,800 kWh, while the loss was estimated, without pulse and solar, to being 17,600 kWh, i.e. a saving of 50%. The total heat consumption for the housing group was 48,500 kWh and thus the pipe loss amounted to 18%, which is slightly larger than expected. This was due to only seven houses being connected out of the planned 12.

The pulses are automatically controlled by a control system that can open and close valves in the individual dwellings, as well as the plant's main valve which is located at the joint flow to the two housing groups. There is thermostatic circulation at the entrance to the two groups and in the two outer houses in the network. They ensure a suitably high flow temperature when the pulse begins to individual houses. The circulation is open during start of pulse and closes when the flow temperature is sufficiently high, typically 55 °C.

For the second heating year (October 2010-September 2011), the saving for the one housing group amounted to 51% and the remaining loss is 11% of the housing group's total consumption. There are now connected 10 houses in the group.

A more detailed analysis shows that the vast majority of savings takes place during periods with only one pulse.

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