CASE STUDIES ORGANIC RANKINE CYCLE
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hat do Otto, Carnot, Rankine and Stirling have in common? An easy ‘starter for 10’ for the physicists, perhaps, but none of
these thermodynamic cycles named after their inventors are as well-known as Mr Diesel’s. Yet, there is a distinct buzz in the building
services world about the latest developments in the cycle, devised by Scottish engineer William John Macquorn Rankine back in the 19th century. The Rankine Cycle is a thermodynamic cycle that converts heat into work. The heat is supplied externally to a closed loop, which usually uses water as working fluid. The Rankine Cycle, based on water, provides approximately 85% of electricity production worldwide. If water is replaced by an organic fluid, thermodynamic efficiency can be greatly increased (see box). Today’s Organic Rankine Cycle (ORC) technology is being used for electricity generation from renewable sources and heat recovery, with a major scheme already completed at BSkyB’s new television studio and office scheme in London, and another under way at BAA’s Heathrow Terminal 2. Both projects are using ORC equipment supplied by Italian-based manufacturer, Turboden. While the company has been developing ORC technology since the 1980s, it is now seeing a widening of interest, given the drive towards lower carbon building services solutions. ‘Traditionally, Turboden has developed projects in which ORC units were suitable for district heating in small- to medium-sized villages,’ says Paolo Bertuzzi, general manager at Turboden. ‘In the last few years, Turboden has started to develop trigenerative projects for large, single public or private buildings whose drive is not only the return on the investment, but also the green vision of the environment.’ Turboden’s ORC unit has been installed
into the UK headquarters of television giant BSkyB in a turnkey design and build project by bioenergy solutions provider, Clearpower. Clearpower has built what is claimed to be the UK and Ireland’s first biomass combined cooling, heating and power (CCHP) system at BSkyB’s new studio and office development in Osterley in west London – Harlequin 1. The main components of the system are housed in a 16,000 m2
energy centre, which
comprises a 5.5 MWth biomass boiler, used to provide up to 2 MW of heat, 2 MW of cooling and to power Turboden’s 1 MWe ORC electricity generator. It is expected to reduce the building’s carbon footprint by at least 20%. ‘You need to have a significant demand for heat for this approach to be technically and
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You need a significant demand for heat for this to be economically viable
economically viable,’ explains Mike Shelly, bioenergy manager at Clearpower. ‘The ratio of heat output to power is around 4:1. BSkyB has both a high demand for heating and a high demand for cooling, so we could use some of the heat produced for cooling as well.’ A crane system will take the biomass fuel (recycled woodchip) into the furnace, where it will heat thermal oil to more than 300°C. This in turn will heat a silicon organic oil in the ORC generator, which will produce electricity, as well as hot water at 90°C as a by-product. This hot water is then siphoned either directly to the heating system or to chillers, which will provide the cooling required for the studios and data centres at the complex. There are alternatives to ORC, of course.
‘There are three options for power generation with biomass systems: gasification, steam turbines or ORC generators,’ says John Heffernan, managing director at Clearpower. On paper, gasification looks the best bet, boasting electrical efficiencies of around 40%, compared to around 19% for ORC and 20% to 30% for steam. ‘There are a number of gasification systems up and running, but it’s not proven technology and there have been reported failures,’ says Heffernan. ‘While an ORC system will perhaps be more expensive and slightly less efficient than steam turbine alternative, it will be more reliable,’ says Heffernan. This is because steam turbines are operating at high pressure, often 4-50 bar,
December 2012 CIBSE Journal 37
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