District heating schemes were once a popular choice for local authorities looking to meet the heating and hot water requirements of large scale housing developments and high rise apartment blocks from the 1950s through to the 1970s. Now, they appear to be making a comeback. Geoff Hobbs explains the role of modern-day centralised energy centres
According to the Department of Energy and Climate Change (DECC), district heating is defined as 'the supply of heat to a number of buildings or homes from a central heat source through a network of pipes carrying hot water or steam'. Heat delivery is achieved via a network of pipes that are laid out and connected into participating properties, so that heat generated in a central location is shared among properties and controlled from a single source.

During the 1980s, local authorities seemed to start shying away from district heating schemes in favour of installing individual boilers in their housing stock.

Problems with leaks or unsustainable heat losses due to long pipe runs led some local authorities to rethink their heating strategies. However, with advances in heating technologies and controls offering greater reliability and efficiency, not to mention the advent of renewable offerings, it appears the time is right for district heating schemes to make a comeback.

Valuable lessons learned

Having learned valuable lessons from early district heating arrangements, both local authorities and private housing providers are now considering district heating schemes as alternatives to individual heating systems where the density of heat usage makes a new scheme practical and cost effective.

With modern equipment helping prevent heat losses and leakages other driving factors behind this resurgence include the need for local authorities to demonstrate their green credentials by delivering heat as efficiently as possible while reducing CO2 emissions at the same time.

System longevity is a key consideration for most stakeholders too. The benefit of modern district heating schemes is that the pipes used to deliver low temperature heat into homes are not specific to the type of technology generating the main source of heat. For example, as well as being suitable for connection to a central boiler room, the same pipes could easily be connected to a CHP unit, a biomass boiler, geothermal heat or a power station.

Having this degree of flexibility is an attractive proposition for local authorities as, when they install district heating pipes based on the technology they happen to be using at the time, they can be rest assured that they will easily be able to upgrade the system in time to incorporate the most efficient and appropriate heating solutions available - for example, renewable and hybrid technologies.

Today's district heating schemes can work by distributing heat generated by a centralised plant room to individual properties via heat distribution units (HDU), which provide both space heating via radiators and instantaneous hot water for the end user. The HDUs available today are specifically designed to keep efficiency to a maximum when fuelled by a centralised source.

For those organisations looking to keep a close eye on usage, the option of a heat meter can prove a useful way of managing costs, monitoring usage and allowing individual dwellings to be charged for the energy they use.

Low supply temperatures with large temperature differences between flow and return are optimal in most modern schemes, ensuring the plant operates at maximum efficiency. The lower supply temperatures also mean that alternative plastic insulated pipe systems can be considered. Careful control of pumping energy and flow rates are essential to maintain the efficiency of a system. The reduced cost of electrical energy used within the scheme for pumping is an added benefit.

Where new build developments are concerned, the Code for Sustainable Homes may also encourage the installation of centralised district heating schemes in the future. This is primarily because of the flexibility of district heating pipework, which presents an opportunity to integrate forms of renewable technology for longer term security but without any disruption to tenants.

Local authorities continue to be the main instigators of such schemes where additional benefits can make the programme more attractive. Issues such as fuel poverty can be helped by such schemes, allowing heat to be pitched at a very competitive level. Reduced annual maintenance of individual gas appliances and the associated issues of gaining access to tenant properties can mean significant financial and time savings for local authorities too, as well as contributing to greater tenant safety.

Great potential for district heat

The DECC estimates that district heating schemes currently provide around 1-2 per cent of the UK's heat demand. However, in the right conditions, district heating schemes could easily supply up to 14 per cent of the UK heat demand, be a cost effective and viable alternative to individual renewable technologies whilst reducing energy bills for consumers.

Given these estimates and the advances that have been made in the technologies suitable for centralised systems, the industry has a fantastic opportunity to put in place a modern generation of district heating systems. The ability of centralised energy centres can prove a viable solution for urban areas featuring a mixture of building types with high heat demand density.

Being able to generate low cost heat while being flexible enough to integrate new technologies in years to come suggests the UK will see more of these schemes being implemented in the near future.

// The author is business development director at Bosch Commercial & Industrial Heating //