How it Works
District Heating (often referred to as Community Heating) is a system for distributing heat generated from a centralized location for residential and commercial heating requirements, such as space heating and process systems. In the Lerwick context we generate the heat through an Energy Recovery Plant (ERP), whereby we burn the local municipal refuse and the refuse from the neighbouring island group of Orkney. This generated heat, in the form of hot water, is then circulated through a series or network of pipes buried through the streets of the main town, Lerwick. This network consists of a Flow and Return main line with connection supply pipes to each individual customer for interface with their internal systems via an internally mounted heat exchanger. This replaces the conventional oil boiler, solid fuel fire or electric storage heaters.
The network of pre-insulated pipes stretches over approximately 40 km across the town of Lerwick, supplying over 1200 customers. The pipes consist of a steel pipe system, with a layer of polyurethane foam securely bonded to the steel and protected by a HDPE outer covering. Each joint is carefully welded using a TIG welding system and then covered with special fittings to maintain the integrity of the pipe network.
In addition to the Energy Recovery Plant, the Lerwick scheme has an oil-fired Peak Load Boiler Station to assist with meeting instantaneous peak demands, typically in the morning and tea-time, and also to maintain the supply during periods of downtime for maintenance and servicing of the ERP. The boiler station has 3 oil-fired boilers, providing a back-up of 14.5 MW in order to maintain and provide security of supply during periods of ERP downtime. This down time period is limited to 40 days per year, spread across two main shutdowns in the spring and autumn. By programming and planning these periods, we can supply the winter load through the burning of waste and avoid expensive oil alternatives.
District Heating plants can provide higher efficiencies and better pollution control than localized boilers. The Lerwick scheme is an example of a visionary, successful and extensive adoption of this technology.
Retired SHEAP General Manager, Neville Martin and retired SIC Environmental Services Team Leader, William Spence, wrote a paper about the District Heating Scheme for the institute of Civil Engineers. You can download the paper here.