District heating has a long history and the technology is now used in many applications and countries. Here, Peter Rose takes a look at the essential part heat exchangers play in distributing thermal energy from central generator stations to client buildings.
A report commissioned by the UK Combined Heat and Power Association and released in early March 2010 has identified a combination of CHP and district heating (DH) as the potential answer to the two biggest energy challenges facing the UK; its rapid descent from energy sufficiency into energy dependency and a parallel commitment to an 80% reduction in carbon emissions by 2050.
The preferred option of the UK government – outlined in its key Low Carbon Transition Plan – is a steadily increasing reliance on electricity for heating and transport. Given the predicted rise in population, the need for unparalleled numbers of new homes and the accompanying growth in essential infrastructure, some experts estimate that this will require a doubling of the current peak electricity demand. Achieving this will involve the building of many, low-carbon or carbon-neutral power stations at previously unheard of rates.
Perhaps unsurprisingly, the UK CHP Association report contends that for this scenario to succeed, more CHP systems will be needed to recycle otherwise waste heat produced during conventional generation and distribute it to communities within a 30 km range of the generating plant. The key to the kind of scheme envisaged will be the use of a carefully-planned DH network to provide the heat and hot water each building will require.
This view, that a CHP/DH combination offers the best chance of maximum energy utilization, is one that is also advocated in a detailed briefing paper by the Institution of Civil Engineers. Conventional coal-fired power stations have an estimated efficiency level of just 35%. Nuclear plants do not fare any better. Consequently, for every 1000 MW of electricity generated, double that amount – in the form of steam used to convert heat into mechanical and then electrical energy – is normally wasted.
Logically, that ‘waste’ energy can be utilized for heating and homes and businesses. That is why the average CHP scheme works at efficiencies of 80% or more. If, instead of being simply vented, this waste energy could be harnessed and put to work it could reduce the country’s heating demand by 25%.
Similar research – reaching more or less the same conclusions – is being replicated around the globe. In temperate countries, the emphasis tends to be on DH networks. In more tropical climes, the opposite is true, efficient district cooling networks are what everyone is interested in. Increasingly, the two needs are also being combined into DHC (district heating and cooling) networks that provide heat in winter for heating and hot water and in the summer to power absorption chillers for air conditioning and refrigeration.
FROM PRE-ROMAN TIMES
What is interesting is that, although it is now very much the flavour of the month, the concept of distributed heat is as old as the hills. The ancient Romans used a hypocaust (literally ‘to heat from below’) to warm their communal baths, villas and even their greenhouses. Raised floors and walls incorporating flues were used to circulate heat from a remote furnace throughout a building. As designs got more ambitious, common walls linking two separate buildings – for instance men’s and women’s public baths – would be used to conduct the air. Some experts claim that the Romans may even have copied the hypocaust from a much more ancient civilisation, since excavations on the Indian sub-continent have also revealed an early form of the system.
Given such antiquity, it is difficult to fathom why an effective energy distribution system should have, effectively, disappeared for a couple of thousand years. Leaving aside a few isolated examples in mainland Europe which survived the demise of the Roman Empire, the DH concept did not really make a comeback until the advent of the industrial revolution.
|Typical district heating layout|
Separate boiler plants and underground piping were used in some English factories in the 1790s. By 1820, this kind of arrangement was becoming quite common. About the same time, recycling of energy also came into vogue, with waste heat from factories being used to warm public baths. There were also several visionary schemes for using the same heat source to warm workers’ homes.
|Carlin Hall development|
A district heating system was used in the enormous glasshouse – christened the Crystal Palace – which became the centrepiece of the Great Exhibition of 1851 in London and at least two steam district heating systems were built in the US in 1853. One, at the US Naval Academy in Annapolis, has been in continuous operation ever since.
Regular film-goers will also be familiar with the characteristic steam jets rising from Manhattan’s streets. What they may not be so familiar with is the fact that the steam comes from a network of pipes connected to a central generating station that has provided heat and power to over 100,000 New York homes and businesses since 1882.
After that brief revival, DH technology seemed to lapse again, driven into redundancy by the ready availability of cheap and plentiful carbon fuels. Countries such as the UK and the US, awash with oil, coal and natural gas, saw little attraction in expensive energy recovery/distribution systems that, generally-speaking, provided an indifferent return on investment.
An added political dimension was provided by the fact that societies with a collectivist bent, such as the old Soviet bloc countries, seemed the most inclined to adopt DH technology. With their emphasis on quantity rather than quality, the DH systems installed behind the Iron Curtain were at best inefficient and at worst frequently out of commission. With significant aid from Scandinavian countries and companies, many of these old systems are undergoing radical overhauls to bring them into the 21st century.
Interestingly enough, one of the world’s first decentralized CHP systems was installed at Battersea Power Station, on the South bank of the River Thames in London. The Pimlico District Heating Undertaking was designed to use waste energy pumped from the coal-fired Battersea Power Station under the Thames to heat 11,000 homes located on the opposite bank. The power station itself was de-commissioned in 1983, but the CHP system has since been adapted to use oil-fired boilers.
Apart from the novelty of its DH/CHP system, Battersea Power Station represented a step change in energy supply that became more or less universal in the years between the wars. Before it was built, London had multiple municipal power stations providing energy to relatively small local areas. The same was true across the Atlantic where most towns of a reasonable size had their own local energy providers.
|Brazed heat exchangers|
The trend to giant networks really became entrenched post-war with the establishment of mega-generating stations and national and provincial grids networking power over extended distances. Today, with environmental considerations ranking alongside commercial ones, many experts feel that trend should be reversed, with the mega-stations supplemented wherever possible by local generation systems linked to DH networks.
|Typical DH sub-station with two brazed heat exchangers|
THE BASICS OF A DH OR DHC NETWORK
The basic DHC concept is simple but, like many simple ideas, extremely efficient. A pre-insulated network of pipes delivers energy from a variety of sources to both domestic and commercial energy consumers. The energy source can be conventionally-fired boilers, CHP plants, waste heat recovered from industry, or renewable sources such as solar, biomass, geothermal and other natural sources of heating and cooling. A DH network, typically, consists of:
- a central heat source
- a heat distribution network (insulated pipes)
- heat exchangers to transfer heat from network to building
- installations in individual unit for heating (heat exchange sub-stations, radiators and controls) and domestic hot water.
Typical district heating layout
Heat exchangers are needed to transfer heat from the primary source to the individual buildings, houses or apartments supplied by the network. In very tall buildings, heat exchangers are also used to maintain pressures and act as breakers. Plate heat exchangers, which offer a combination of compact size, low weight and high thermal efficiency, represent the preferred heat exchanger solution for district heating and cooling systems around the world.
The traditional gasketted plate heat exchanger, which can be opened, is specified when mechanical cleaning of the unit is required. Brazed heat exchangers, in which gaskets are replaced by a brazed seal, are used when a cost-effective and more compact solution is preferred. These two types are employed mainly in DH substations, for tap water heaters and in space heating loops. Where a system involves high temperature and high pressures, all-welded plate heat exchangers are the best option.
To supply heat and hot tap water to a tall building dozens of heating units are combined in several large heat exchanger substations within the building and connected to the district heating network. This is crucial in maintaining pressure in heating networks and pipework. Providing heat and hot water for individual units is generally the job of a small substation.
Generally speaking, DH represents the lowest whole life cost option for heating provision for an apartment block, a shopping centre or a complete district or city. It can be up to 70% cheaper than a direct oil-fired boiler, depending on the fuel source employed. Large central boilers or CHP plants not only provide economies of scale compared with individual units for each household but also reduce the risk of leaks and explosions. A well-planned DH system provides unparalleled opportunities for energy savings and emissions reduction, especially as modern boiler plants incorporate efficient gas flue cleaning systems.
SOME CASE HISTORIES
Without their own natural resources to draw on, countries such as Sweden and Denmark have made DH and CHP a central plank of their energy policies for several decades. As previously mentioned, former Soviet bloc countries also employed DH technology for their large municipal housing complexes, albeit with much lower rates of efficiency and success. The three boxes illustrate three very different examples.
Peter Rose is Marketing & Communications Manager with Alfa Laval UK.Email: firstname.lastname@example.org
Upgrading DH in Russian cities
In Russia, most of the country’s district heating plants and networks are 40 years old. Consequently, three large projects have been in progress since 1997 to improve and upgrade district heating in dozens of major cities such as Volgograd, Kaliningrad, Vladivostok and Kazan – where winter temperatures frequently reach -50°C and below.
Alfa Laval has supplied plate heat exchangers (PHEs) to replace old shell-and-tube units in central and individual heating substations to upgrade these DH networks. Significantly smaller and more efficient than the old units, the PHEs have helped these cities achieve substantial energy savings; 23% on average for space heating and 13–25% for tap water heating. Since they have also reduced the volume of pipe-work required for each installation, they have also limited the risk of leakage.
|Heat exchangers in a Moscow building|
Burj Khalifa, Dubai
From -50°C to +50°C in Dubai
At 828 metres high, the Burj Khalifa is the world’s tallest building, housing thousands of luxurious apartments and offices, as well as a hotel designed and decorated by Giorgio Armani. Since it is located under the scorching desert sun of Dubai, where temperatures often reach + 50ºC, Burj Khalifa obviously requires massive air conditioning capacity. For this purpose, the complex has its own district cooling (DC) plant, and employs an innovative cooling system based on a thermal ice store.
Tonnes of ice slurry are produced during off-peak hours and stored in a tank. This stored cooling energy is transferred via plate heat exchangers via an internal district cooling network, to the entire indoor air conditioning and tap water networks.
The ice storage solution requires a fraction of the space needed by conventional cooling equipment which was a major consideration for the designers and builders of the huge Burj Khalifa. The ice provides back-up to the system in the event of a chiller failure. A total of 30 large plate heat exchangers are used for the air conditioning system, five for the tap water cooling system and eight for the swimming pools in the complex.
Green in Ireland
In the Republic of Ireland, the Dundalk Sustainable Energy Zone is an area of around 4 square kilometres which will encompass housing, schools, hospitals and businesses and act as a focus for sustainable energy initiatives.
One of the first developments within this area is Carlin Hall (see photograph on page 46).; a mix of two, three and four bed apartments, semi-detached and detached houses all connected to a central DH system. Each household takes heat and hot tap water via 4000 metres of pre-insulated steel pipe-work connected to a central biomass boiler burning a wood chippings supplemented by natural gas to produce 1.2 MW of heat. A mini sub-station located in each dwelling gives Individual householders control of water and heating temperatures. It also houses a heat meter to measure the amount of energy being consumed in each dwelling.
With ambitious targets of 90% lower emissions, 30% more energy efficiency yet 30% lower energy costs, Carlin Hall will set the benchmark for future energy-efficient housing.