Telehouses demand high-reliability combined cooling and power20 August 2001
Two new data and telecommunications switching centres in the UK are being equipped with CHP systems close coupled to absorption chillers. They are setting new standards for reliability
The London Internet Exchange, which handles more than 90 per cent of the UK’s Internet traffic, is switching the equivalent of 400 000 emails every second at peak periods and says that the volume of traffic is doubling every nine months.
Major new broadband networks are needed to cope with this traffic growth. One such network is the 2100 km fibre optic cable system being constructed in the UK by telecoms company 186k Limited.
186k has awarded a multi-million pound contract to Lattice Energy Services (LES), in partnership with Brotherhood Aircogen (Aircogen), to provide secure, cost effective energy to two new ‘telehouses’ – telecommunications switching centres – at Hemel Hempstead and Bristol.
Both sites presented the LES/Aircogen team with unique problems for which different solutions have been produced. Both are based on Aircogen Nimbus CHP (combined heat and power) equipment, which provides the electrical power. This is close-coupled to absorption chillers to provide air conditioning, thus making the systems combined cooling and power (CCP) plant (Figure 1). Both systems set new standards of efficiency and reliability in an energy-hungry market.
The primary energy source for both sites is a number of natural gas engines (Figure 2) configured to meet the peak demand even with one engine on standby. This ‘n+1’ planning concept means that planned maintenance and minor breakdowns can be accommodated without needing to use any emergency back-up power from the electrical grid and/or uninterruptible power supplies (UPS).
Security and reliability of power supply and air conditioning is vital for telehouses in order to maintain their round the clock, 365 days per year operation. Any failure of the electricity or cooling supply, even for a few seconds, could have expensive consequences in terms of cost and reputation.
Whilst the electricity grid is generally reliable it is prone to failures during severe weather and increasingly suffers technical upsets. The grid on its own has therefore never been deemed sufficiently reliable for telehouse operation. Another factor is grid power quality, which can vary considerably – to the detriment of sensitive unprotected electronic equipment.
Traditionally, the solution to the reliability problem has been to back-up the grid with an uninterruptible power supply (UPS) – either rotating (where a means of energy storage is driven continuously by a diesel engine, the diesel taking over in the event of a mains failure) or a static system based on batteries, with diesel engine generators used for longer outages and to recharge batteries.
Both systems have similar problems: the storage of energy of necessity involves the inefficient use of energy to create and maintain the store; the diesel generation equipment is idle for most of its life and will suffer reliability problems unless frequently tested and maintained at appropriately high cost; and the infrequent use of the equipment and high cost of installation and use represent a poor financial investment.
These problems have been avoided at both Bristol and Hemel Hempstead. In addition to the reliability inherent in having ‘n+1’ CHP units, further back-up is provided at the Bristol site by a grid connection capable of providing 100 per cent of the site’s peak load. At Hemel Hempstead 50 per cent of the electrical back-up is provided by the grid and 50 per cent by SUPS (static uninterruptible power supplies). The combination of CHP, spare CHP and back-up by UPS and/or grid, means that the design level of power supply reliability achieved by the LES/Aircogen system exceeds 99.999 per cent.
Combined cooling and power
Telehouses require electrical energy and air conditioning (cooling) in broadly equal measure. These are usually delivered at poor overall energy efficiency due to the high energy cost of electricity-based air conditioning.
Typically, around 30 per cent of the electrical energy used by a telehouse drives cooling and air conditioning equipment. In a CCP system, waste heat from the gas engines is converted to cooling through absorption chillers (Figure 3).
Absorption chillers for air conditioning are proven technology used worldwide, particularly in Japan and Asia. The Lattice Group’s technology centre at Loughborough has operated such a system for five years, during which time a reduction of around 850 tonnes/year of CO2 has been achieved, together with significant cost savings.
Operating on the lithium bromide cycle, the CCP plants use water as the refrigerant (no ozone depleting refrigerants) and provide an economic, reliable and environmentally friendly means of converting waste heat into cooling. In the 186k telehouse CCP schemes the absorption chillers are configured so they can operate independently of the gas engines, utilising oil as the backup fuel to gas. This provides maximum system reliability, ensuring that both power and cooling can be delivered in the required quantities even in the event of interruption of the natural gas fuel input.
The use of absorption chillers displaces around 22 per cent of the electricity load required by a conventionally powered and cooled telehouse – the amount typically needed to operate vapour compression chiller systems. The ratio of cooling and power produced by CCP very closely matches the site requirements.
Tax and environmental benefits
There are significant environmental benefits associated with on site generation using natural gas, which is the cleanest of all the fossil fuels, coupled with using the waste heat for cooling.
Imported electricity is delivered at an average efficiency of between 30 and 40 per cent whereas a typical CCP scheme will have an overall efficiency of 80 per cent or more. In addition, it reduces CO2 emissions by a minimum of 30 per cent compared with conventional systems – a better level of reduction than the ‘Kyoto target’ – and contributes to a reduction in SOx and NOx emissions.
The exceptional environmental performance of the CCP system secures a number of tax advantages for UK operators, including Climate Change Levy exemption and enhanced capital allowances. Similar tax benefits accrue in other European countries, all of which are seeking to encourage energy efficiency through tax incentives.
The tax benefits, the reduced electrical connection costs and the reduced cost of standby generation and electric air conditioning combine to ensure that the system provides significantly lower lifetime costs for energy supply than is possible with conventional arrangements. Figure 4 shows typical costs for a 15MW site using UPS systems and diesel generators with electric air conditioning compared to the same site using a CCP solution.
There is an additional benefit of CCP systems which is of particular relevance to the fast-moving telehouse market.
Telehouse operators are under pressure to take advantage of the market conditions by getting their new sites operational as quickly as possible. The connection to the electricity grid can often be a determining factor in how long this takes, with reinforcement of the existing electricity networks taking up to two years. Lattice Energy Services/Brotherhood Aircogen can provide a scheme that will meet all the energy requirements for a site in less than 34 weeks from conception to commissioning, thus allowing the customer to begin trading (and making money) sooner.
The modular nature of the system means it can be expanded to meet growing load, making it possible for a telehouse to match investment in power production with its needs as the business develops and more load comes on-stream.
Lattice Energy Services (LES) is part of Lattice Group plc, formerly part of BG Group plc. It provides energy infrastructure and consultancy services.
LES is managing the entire installation of the projects at Bristol and Hemel Hempstead, providing the energy infrastructure and connections to the plant.
The systems will be owned and operated by LES, which has a long term contract with 186k – including performance and reliability guarantees – to provide electricity and cooling. Because LES is funding all the capital investment there is no impact on the 186k balance sheet.
Brotherhood Aircogen is a subsidiary of Peter Brotherhood Ltd which specialises in the design and manufacture of CCP and CHP systems. Its role at the Bristol and Hemel Hempstead telehouses is to provide engineering services, from design and construction of the CCP plant through to ensuring its reliable, efficient and cost effective long term operation.
Both sites are scheduled to enter commercial operation in 2001 and will be the first of their kind in the UK telecommunications sector.