CIBSE Case Study: District Energy Scheme Providing Low Carbon Heating & Cooling
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Article from the August 2011 edition of the CIBSE Journal by Andy Pearson
The development of Stratford in east London has delivered not only the Olympic Park but also a district heating system powered by two innovative energy centres. Andy Pearson gets an inside view.
There is a year still to go until the start of the London Olympic and Paralympic Games, but already the legacy of the 2012 Games is evident for one housing development in east London. An energy centre, built to provide an efficient, low carbon heating and cooling system across the Olympic Park site for the Games and the long-term regeneration of the area, is already supplying a Genesis Housing scheme of fi ve residential blocks, a care unit, business space and retail units situated adjacent to the site.
The district energy scheme provides low carbon heating and cooling from two state-of-the-art energy centres, which incorporate combined cooling, heat and power (CCHP) systems and biomass boilers. The centres, known as the Kings Yard and Stratford City, and the associated network of heating and cooling mains have been designed, built and financed at a cost of £113m by utility provider Cofely, a division of GDF Suez, following competitive tender.
The Kings Yard energy centre, to the west of the Olympic Park, is the larger of the two schemes. It will supply power to the national grid, along with heating and cooling to the Park's permanent and temporary Olympic venues and the Olympic Village. The Stratford City energy centre will also supply electricity to the grid, but its heating and cooling energy will be supplied primarily to Westfield shopping centre and the associated commercial and office developments currently under construction at the entrance to the Park.
Developing the largest district energy scheme to be built in the UK, on time and with sufficient capacity for the Olympic Park and the adjacent retail park, was only part of the task. Equally importantly, when the Games are over the energy centres will continue to be developed and run by Cofely's specialist business unit, Cofely District Energy, for the next 40 years to provide the Park's legacy buildings and surrounding developments with low carbon heating and cooling. The challenge is that, whilst the energy demands of the Park's venues during the Games are known, the future energy demands of the site's legacy buildings are less well defined.
Current equipment installation and additional equipment planned for the post-Olympics legacy period, and capacities (MW)
The approach has been to design and build the energy centres in a modular format to enable plant to be added in the future, once the legacy loads are known. The utility had two years from the start of construction to build the two energy centres and the sitewide network of 16km heating and cooling pipework.
Both energy centres are housed in buildings designed by architect John McAslan & Partners. The centres are large, brown rectangular boxes, wrapped in a mesh of pre-rusted, perforated cladding panels with a 45m tower at one end housing the boiler flues. The rusty facade was not installed until the key items of plant had been positioned on the building's steel frame. To enable future plant to be installed, sections of the building's cladding have been designed to be easily removed.
Inside the giant Kings Yard energy centre, the modular approach to the scheme is immediately apparent. Adjacent to the two, huge 20 MW dual-fuel gas/oil fired boilers is the space for three additional boilers. All the pipework and flues are in place so that boilers can be added in the future with the minimum of disturbance, while the system remains live. Similarly, adjacent to the 3.3 MW gas-fired combined heat and power (CHP) engine, pipework connections are already in place in four empty bays for further units, if required.
In summer, when the demand for heating is less, heat recovered from the CHP units can be used to drive a 4 MW absorption chiller. Even if the absorption chiller is not running, cooling can still be provided by two, 7 MW ammonia-based chillers. Again, space has been allocated for an additional future chiller. The chillers reject heat through five roof-mounted cooling towers.
During the Games the main demand for cooling will come from the International Broadcast Centre and the Handball Arena. After the Games, the Handball Arena is set to become a community sports centre, but the big and, as yet, unanswered question is: what will happen to the Broadcast Centre, particularly in terms of utilising the installed cooling capacity?
To ensure cooling can be supplied efficiently, even under light load conditions, the chilled water circuit includes a giant cylindrical chilled water buffer vessel. The 750 cu m vessel increases the capacity of the chilled water system by 4.7 MWh, so that when loads are low the ammonia or absorption chillers can run uninterrupted, charging the vessel. A similar system operates on the hot water circuit, with the 27.5 MWh capacity buffer vessel intended to allow the uninterrupted operation of the CHP engine and the system's giant boilers. A third tank contains treated make-up water for the hot and chilled water system. The enormous tanks are situated outside the building, adjacent to its eastern facade.
The plant's current installed capacity is 46.6 MW heating, 18 MW cooling and up to 6.68 MW of electrical power, depending on loads. When all the plant is in place the energy centre will have the potential to generate up to 122.5 MW heating, 25 MW cooling and 10.02 MW of electrical power.
In contrast to Kings Yard, Stratford City energy centre has no spare cooling capacity and less future capacity for heat-generating plant because the energy centre was built primarily to meet the established cooling and heating loads of the new Westfield Shopping Centre and its associated office development. This energy centre features two 3.3 MW CHP engines; a 4 MWabsorption chiller; five 7 MW ammonia chillers; and two 20 MW dual fuel gas/oil boilers. These give it a current output of 46.2 MW of heat and 39 MW of coolingand up to 3.34 MW of electrical power, depending on loads.
The Stratford City energy centre also has the capacity to add another 26.2 MW of heat with an additional boiler and two CHP engines. Some redundancy has been built into the centre's total capacity to enable the systems to run uninterrupted even while items of plant are off-line for maintenance. Between them the two energy centres have the potential to supply a total of 194.9 MW of heating, 64 MW cooling and 30 MW electrical power.
There is an agreement to achieve a reduction in carbon emissions of 20% in 2012 as a result of the CHP services, rising to 30% in 2013. When fully operational, the scheme has the potential to save up to 12,000 tonnes of CO2 a year, compared with conventional energy supplies.
While the energy centres have been designed to operate independently, two giant heating mains link the two buildings. This enables the heating plant to be run efficiently under low-load conditions. Currently the heat loads are supplied from Kings Yard; however, as more venues are completed and heat loads ramp up as the Games approach, more capacity will be progressively brought online until each centre is operating at design capacity.
Substations are strategically located around the network to enable heating and cooling to be supplied to the venues. These skid-mounted units were prefabricated at the Milton Keynes works and comprise one or two heat exchangers, depending on whether heating and/or cooling is being provided, with controls and pipes attached. A secondary circuit transfers heat from substation to the consumer. A total of 75 substations have been installed to serve both the permanent and temporary Olympic venues and legacy schemes.
The network operates as a variablevolume, constant-temperature circuit, with the volume varied by altering the speed of the circulating pumps. The system is set up to maintain a differential pressure at the index point, which is the point in pressure terms furthest from the energy centre; in reality this means there is sufficient pressure difference between the flow and return mains to push the heating or cooling water through the heat exchanger at this point.
In addition to the two McAslan-designed buildings, the Kings Yard scheme also incorporates a Grade II listed Edwardian building, which has been renovated. This is the Olympic Park's only retained building; it is situated adjacent to the energy centre and will house a 3.5 MW biomass boiler and woodchip store as well as provide the space for a future visitors' centre. The boiler is due to be operational by the end of 2011, and a wood-chip supplier is currently being sought that complies with the Olympic Delivery Authority's (ODA) sustainability criteria. There is space for a second biomass boiler to be added in the future. A 16 km network of buried pipesdeliver the district heating and cooling throughout the Olympic Park (see box).
The heating mains operate at temperatures of 95C flow 55C return, while the chilled water mains operate at 6C flow and 12C return.
A scheme of this scale does not come cheap. The cost of the new energy centres and district heating mains have been financed by Cofely, who will recover their investment through the long-term operation rights of the new infrastructure.
Currently, energy demand is starting to ramp up as preparations for the Games gather pace. The Games and the Paralympics will provide an eight week demand peak. After the Games, demand will drop for a year or so while the temporary venues are removed, the remaining venues undergo their conversion into legacy buildings and the athlete's village is transformed into homes.
The first legacy scheme likely to be constructed after the Games will be a housing development, built on the site of the temporary basketball arena. Critically, the mix of development slated for this area has changed. Originally the plan was to develop a high-density housing scheme with 12,000 homes located mainly in high rise apartment blocks. However, the proposals have changed so that the scheme now has 5,000 fewer occupants and a higher proportion of lower-density family housing. The extent of district energy service required, and the subsequently high heat losses, mean that low-density housing is, traditionally, unviable from a district heating perspective.
However, options are being examined to ensure that, when the scheme is connected, it will be in the most efficient way possible. It would be ideal if new developments with a high heat load are attracted to the site after the Games. The energy centre scheme is heat led, which means the utility needs to sell heat to make the CHP and biomass boiler systems viable to operate, and to reduce the carbon intensity of the heat supplied. The ideal businesses to set up on the site would be hotels, leisure and healthcare facilities because they have high hot water loads. It helps, too, if the loads are clustered together to minimise the pipe network; it is also beneficial if the loads are from a diverse range of businesses to even out demand throughout the day.
The project has been helped in its mission to sell heat by the Olympic Park being declared "an area of exclusivity". This designation entitles the utility to be the sole provider of heating and cooling on the site; a "price control formula" has been put in place to regulate the price at which heat can be sold to ensure it costs less to end-users than traditional means.
Originally the project business plan included the sale of 20% of the energy centres' heating capacity outside the Park's boundary. However, following changes to the legacy plans, this figure is likely to increase. The heat supplied to the Genesis Housing Group's scheme is the first scheme outside the Olympic Park boundary to benefit from the district energy network.
Pipework: Installation of the heating and cooling mains
One thing was for certain: the 16km network of district heating and cooling pipework that link the two energy centres with the Olympic venues had to be in place for the Games. This achievement was only possible in the two years from commencement of construction to operation because the entire installation was modelled in 3D (see Design: 3D Modelling).
There was insufficient time to wait for the venues to be constructed, and for the site to be landscaped, before installing the heating and cooling mains. Instead the mains were installed in sections as areas became available. It was a challenging operation with mains passing beneath railway tracks and over bridges to reach all the Park's venues. Over time these sections were gradually joined as further sections of the site became available, until the network was completed.
The mains are constructed from pre-insulated carbonsteel pipes comprising 50mm of polyurethane insulation enclosed in a polyethylene protective outer sleeve. The pipes have a built-in leak detection system. The pipes were installed in 12m lengths, at diameters up to 400mm, and welded together. Once the leak alarm cables were joined, a muff was wrapped around the pipe joint and insulation injected into the void created by the muff.
The pipes are buried so that the top of the pipe is at least 1m below surface. By using buried pipework, network losses are low, with a temperature drop of around 1C per kilometre. The advantage with the site is that there were very few buried utilities to avoid. However, on the down side, because the site was still being reconfigured the ground was not always at the finished level. As a result, some pipes over 2m had to be installed above the ground on temporary supports until the final ground level was established.
Design: 3D modelling
The project design team used 3D packages to develop the scheme and build the energy centres virtually, before construction commenced.
The design was developed using a variety of software packages including AutoCAD 2010, AutoCAD MEP and 3D CADduct. To assemble the energy centres, the steelwork layout was imported from the fabricator. However, the existing Edwardian building, housing the biomass boiler, was modelled using 2D drawings in AutoCAD before being imported into a design package. The scheme was drawn in AutoCAD with CADduct running alongside so that the designers could access the CADduct library of pipe fittings and other services. The larger elements of plant, such as the boilers, were modelled by the CAD team and imported into the design.
To render the model, Navisworks was used. The designers used the rendered model to view the design and check for any clashes visually or run clash-detection software. The model was also used to work out delivery routes for the installation of future legacy plant. Once the designers were happy with a scheme, it was published as a model along with dimensioned sections and plans. These are produced as PDF files and published on a Projectwise database so that the sub-contractors can access the current drawing.
Along with the energy centre, the pipework distribution network was modelled in 3D by importing ground-level data and converting this into a model of the site. The model has all 75 substations, to enable teams working on individual buildings to model the connection to the heating mains. This model has been uploaded to the Olympic Delivery Graphic Information System model of the site.
A 3D image showing, in purple, the gantry containing the biomass boiler flues which connect the exisiting building to the new Olympic Park energy centre. Roof-mounted cooling towers are shown in red, and the buffer tanks at the rear in brown.