About the Lerwick Scheme
Currently all the heat for the Lerwick District Heating Scheme is provided by the Energy Recovery Plant which is run by the local authority.
The Lerwick District heating network consists of two pre-insulated pipes, one is the supply pipe with water at 95°C and the other is the return pipe. The return pipe goes back to the energy recovery plant for reheating. District heating works more efficiently if the return temperature is as low as possible. Currently the average return in the winter is about 55°C and during the summer is about 60°C. The reason for the higher return in the summer is that most of the load is to large consumers such as the hospitals that were originally designed for a higher return temperature. As new buildings such as Mareel and the Anderson High School come on with lower designed return temperatures nearer to 40°C our overall return temperature should also come down. The pressure difference between the flow and return pipes is a minimum of 0.5 bar at the extremities of the scheme.
When the consumer opens a tap or turns on the heating the heat exchanger unit opens up allowing water to flow through the unit. The hot water and the radiator circuit have separate plate heat exchangers. These are made up from a series of metal corrugated plates, usually from stainless steel, which forms chambers for the water to pass through. As the incoming cold water or radiator system fluid passes through these chambers, the high temperature district heating water passes through alternative chambers and the heat is transferred across. By this means, the network water never enters the domestic system and merely transfers the heat energy into the relevant system on demand.
The heat transfer is practically instantaneous. Cold mains water is heated from 10°C to 50°C normally (or whatever temperature the consumer has it set at) and as long as the mains pressure is adequate, the flow can be continuous and at high pressure. In most cases the capacity of the unit is sufficient to heat all the incoming water but in a few cases with high supply pressure the tap(s) may need to be turned down so as not to exceed the heating capacity. In all but a few cases the need for a hot water storage tank is removed for houses and other properties that do not need an excessive demand or a reserve buffer for security of supply. Over the ten years of operation the supply has rarely been interrupted. The main interruptions have been from power cuts, which usually affects the consumers as much as us. We have a back up generator to enable some flow but it can take a few minutes to come on line fully. As far as the heating circuits are concerned there will be sufficient heat stored that our supply would need to be off for an hour or more before it would be noticed.
In practice the district heating system has proved to be very controllable. The exchanger capacity can normally bring a house up to temperature very quickly. Using Thermostatic Radiator Valves (TRVs) on radiators allows the flow to reach an equilibrium avoiding fluctuations in temperature.
The energy used by a consumer is measured by an ultrasonic meter which has no moving parts and therefore unlikely to seize up. We periodically send units away for calibration tests to check reliability. The meter measures the flow and the temperature in and out and then computes how much heat the customer has taken out of the system reading in either kWh or MWh. Through simple interrogation the customer can read the temperatures, flowrate and heat demand at any given time. The meters are fitted with a radio meter and every quarter (or more often if required) a signal is sent out from the Boiler House Control Centre, requesting the meters reading to be relayed back to the office. This removes the need for estimates or customers having to send in meter readings and also allows us to check that the customers units are working satisfactory. Through this system we can provide accurate billing to the customers.