Combined Heat & Power

Demand Side Services

Energy Efficiency

Heat Networks

Citigen tri-generation scheme

The Citigen Tri-generation Combined Heat and Power System supplies heating and cooling to a number of well-known buildings in the City of London and was one of the first such systems to be developed in the UK

Citigen tri-generation scheme | Building CHP

The Citigen CHP system has been operating in the City of London since 1993 and comprises a CHP central energy centre near Smithfield and a11 km district energy network in the north-west part of the City supplying public and private properties with heating and cooling.

The system supplies heating to the City Corporation’s properties, including Guildhall, Smithfield Market and the Barbican Centre. Private customers are also supplied, including offices and residential accommodation. Cooling (as chilled water for air conditioning) is provided to Corporation properties and private customers from a separate parallel pipe network. Electricity generated is exported to the local distribution grid and sold through Citigen’s parent group.

The plant provides efficient, cost effective, low carbon heating and cooling 24 hours a day and achieves savings of 11,500 tonnes of CO2 a year.


The idea for an energy network linking several of the City of London Corporation’s buildings to be served from a central energy plant was originally conceived by the City Corporation in the late 1980s. The concept was considered an attractive proposition to a CHP developer on a commercial basis as:

  • The City area offers a high load density with a good mix of buildings, new and old
  • The City Corporation's own buildings could provide the initial heating, cooling and electrical loads
  • Future loads would come from the private sector and other public authorities as the system developed
  • A site was available for the energy centre
  • Existing subways and basements were available for pipe and cable routes, and
  • The system would benefit from the powerful backing of the City Corporation itself

Following detailed studies by Atkins, expressions of interest were sought from interested parties, and British Gas and Utilicom Ltd were selected to form a joint venture, Citigen, to undertake the role of ‘ESCO’ (Energy Services Company) to develop and operate the system. Over the years Citigen has undergone several changes of ownership but since 2002 has been a wholly owned subsidiary of E.on UK plc, and now forms the centrepiece of E.on City Energy Solutions division.


The basis of the partnership between Citigen (the ESCO), and the City of London Corporation (the Local Authority) is a long term 30 year ‘co-operation agreement’ which binds the parties to work together to develop and expand the system. Under the agreement, Citigen is responsible for the design, development, financing, and operation of the scheme and carries the commercial risks; the City Corporation provided the ‘anchor’ loads, gives support and planning advice, and encourages private customers to take supplies.

Other agreements between the parties include heating and cooling supply agreements to individual buildings and pipework and cable licences. The City has also leased original boiler plant at two sites for system back-up purposes, and leased the building for the energy centre.

Early challenges

The site identified for the energy plant near Smithfield Market lies in the heart of the lively Farringdon area and presented a number of challenges. It stands behind the façade of a former cold store which had to be retained, had very restricted access through the front façade only, and stood directly over a railway tunnel.  Some areas of the building were demolished whilst those parts over the railway were left.

The CHP plant originally selected comprised two 350 tonne V18 marine diesel engines, adapted to run on high pressure gas. These were capable of producing 30MW of electricity and 32MW of heat.

Commissioning these units proved an extremely lengthy and difficult process. The two engines were oversized for the heat load then available and excess heat had to be dissipated from 10 roof mounted cooling towers.

The City’s streets are heavily congested with other utility services and this made routes for buried pipework difficult. The majority of the pipework and cables were able to make use of existing subways, basements and car parks for ease of installation and access, but around 30% still had to be buried directly under roads.


Delays in commissioning the plant and granting licences for pipework unfortunately led to contractual difficulties after only a few years, but these difficulties were successfully resolved and working relationships have been good since then. Subsequently the plant proved unreliable and expensive to maintain and following reform of the electricity market and increases in gas prices became increasingly uneconomic to operate.

Replacement with new plant

In 2013 the decision was taken by Citigen to replace the original CHP plant with more modern units and a thermal heat store better matched to the available heat loads. This represents an investment of over £25 million. Following an extensive tender process Citigen awarded CHP specialist Edina Group the contract to remove the existing plant and supply, install and maintain two new gas fuelled MWM TCG 2032 V16 high efficiency CHP sets. These each produce 4.3MW electricity and 4.1MW of heat. Space has been allowed for a further two sets at a later date when the heat demand justifies it.

Heat is recovered from engine exhausts, turbochargers, jacket cooling, and lubricating oil and transferred via heat exchangers to the district heating network. Surplus heat is stored in a 330 m3 thermal heat store for draw off overnight when the engines are not running. Back-up boilers at the energy centre and two other locations totalling 25MW provide resilience and top-up heat to the network if required.

The works were complicated by the need to maintain heating and cooling supplies to customers throughout the period. With the old engines removed, preparatory works were undertaken in advance. A new slab was cast 8 metres below street level supported on concrete columns to carry the new engines and their acoustic enclosures and services.

Site logistics was a major challenge. Internal demolition work, alterations and construction ran concurrently with Edina’s equipment installation. Detailed co-ordination and phasing of work was essential to meet the programme and health and safety requirements. Due to the amount of equipment arriving on site, road closures were unavoidable. Each of the 53 tonne CHP sets was delivered in three parts – bedfame, engine and alternator – and reassembled on site. The construction of the engine cells, pipework, cabling and the delivery, offloading and installation of equipment placed a heavy reliance on scaffolding and lifting equipment.

Until 2013 chilled water was produced mainly by two 5.6MW Trane absorption chillers but these were decommissioned and replaced by a new 3.3MW high efficiency electric chiller under the redevelopment scheme. Total cooling capacity is currently 6.3MW. Although all chilled water is currently generated by electric chillers, a new absorption chiller may be added in the next year or so and new sources of energy (renewable, waste, additional storage) are being actively investigated to improve the carbon savings credentials of the network. 

Distribution Network

The main part of the heating and cooling distribution network was completed by 1998 and at that time served 13 properties. This area of the City is fortunate in having many subways, basements and car parks that could be used for pipework, which was easier and cheaper than burial under roads. The City has granted Citigen licences which set out the conditions for installation and subsequent use and maintenance. No charge is made for these licences.

Pipework is largely pre-insulated welded steel with either HDPE or galvanised outer casing, up to 350mm diameter. Leak detection is provided but not currently used. Isolating valves at key locations allow sections of pipework to be isolated for maintenance.

The variable flow district heating system operates at up to 105⁰C and is pressurised to 4 bar by a pressurisation unit at the energy centre. A back-up pressurisation unit is installed at the Barbican Centre for resilience. In an extreme situation the system can be operated without the energy centre if necessary, using back-up boilers at Guildhall and Bastion House.

The variable flow chilled water system operates at 6⁰C with return about 9-12⁰C and is also pressurised to around 4 bar. Back-up pressurisation is not provided for this system as all cooling plant is located at the energy centre.

Customer connections

At each customer site the incoming Citigen heat supply is connected to building’s main heating circuit via a compact plate heat exchanger. This saves space, is silent in operation, and avoids the need for a gas supply, boilers, flue and ventilation. A heat meter records the energy usage from which charges are calculated. Existing boilers may be retained for back-up if required. A similar connection arrangement is used for cooling supplies, and avoids the need for on-site chillers and cooling towers.

Future development

New connections up to 100-150 metres from the energy network continue to be added. In the case of new developments these are largely driven by a planning obligation to connect to the system and the need to comply with the GLA’s Spacial Development Strategy. Citigen will normally make a competitive offer which demonstrates a saving compared with on-site boilers and chillers over the plant lifetime.

Although there has been no strategic expansion of the network since 1998, it was always anticipated there would be further expansion into surrounding areas, and plans are being drawn up to achieve this. Opportunities have been identified in the areas to the south and east of the existing system. It is expected that a HNDU feasibility study will be undertaken as the first step.

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