Overview of the Lerwick District Heating Scheme

The Lerwick District Heating Scheme is a centralized heating system that provides heat to homes and businesses in the Lerwick area through a network of pre-insulated pipes. The heat is generated by the Energy Recovery Plant (ERP), which is operated by the local authority. This system efficiently distributes heat and has been designed to meet the heating needs of the community.

How the System Works:

1. Energy Generation at the ERP: The Energy Recovery Plant generates heat by burning municipal refuse and waste from neighbouring areas, including Orkney. This heat is transferred to water, which is circulated through the district heating network.

2. Pre-Insulated Pipes: The network consists of two pre-insulated pipes:

   • The supply pipe carries water at 89°C to the customers.
   • The return pipe carries the cooled water back to the ERP for reheating.

3. Efficiency Considerations:
   The system works most efficiently when the return temperature is as low as possible. In winter, the average return temperature is about 55°C, while in summer it rises to 60°C. This increase in summer is due to higher temperature requirements from large consumers such as hospitals, which were designed to operate with a higher return temperature. However, as new buildings like Mareel and Anderson High School are added, which require lower return temperatures (around 40°C), the overall return temperature should decrease.

4. The Heat Exchanger: Each customer has a heat exchanger unit installed. When a customer opens a tap or turns on the heating, the heat exchanger activates and allows water to flow through it. There are separate plate heat exchangers for the hot water and the radiator circuit. These plates, typically made of stainless steel, have corrugated surfaces that create chambers for the water to pass through.

   As the incoming cold water or radiator fluid flows through the chambers, the hot district heating water flows through adjacent chambers, transferring heat across without the two systems mixing. This design ensures that the district heating water never enters the domestic system.

5. Hot Water and Heating Control:

   • The heat transfer process is nearly instantaneous, rapidly heating the cold mains water from 10°C to 50°C (or to the consumer's desired temperature).
   • If the supply pressure is adequate, the water can flow continuously at high pressure. In most cases, there’s no need for a hot water storage tank as the system can provide instant hot water.
   • Some properties with high supply pressure may need to adjust their taps to prevent exceeding the heating capacity.

6. System Reliability:

   • Interruptions to the supply are rare. The most common cause is power cuts, which affect both the consumers and the Energy Recovery Plant. However, the plant has a back-up generator to maintain some flow during power cuts, although it may take a few minutes for the generator to fully come online.
   • The system is highly controllable, with thermostatic radiator valves (TRVs) used to maintain a stable temperature without fluctuations.

7. Measuring Energy Use: Energy usage is tracked with an ultrasonic meter that has no moving parts, making it durable and unlikely to seize up. The meter measures the flow and temperature in and out of the system and calculates the amount of heat consumed by the customer in either kWh or MWh.

   • Radio Meter Reading:
     The meters are fitted with a radio meter that sends a signal to the Boiler House Control Centre every quarter (or more frequently if needed). This allows for accurate remote reading of the meter and eliminates the need for customers to manually submit meter readings.
   • Customer Access to Data:
     Through simple interrogation, customers can view data such as the temperatures, flow rate, and heat demand at any given time.

8. Accurate Billing: The system enables accurate and timely billing, as the consumption data is automatically transmitted. This ensures that the bills are based on actual usage, rather than estimates.

The Lerwick District Heating Scheme offers an efficient, environmentally friendly, and reliable heating solution, which has been designed with both energy efficiency and consumer convenience in mind. The system's advanced technology, including real-time data collection and automatic meter reading, helps ensure that customers benefit from an accurate and seamless service.