CARBON NEUTRAL PROPOSAL ORGANIC RANKINE CYCLE
ORC’s path to carbon neutral
For ‘nearly zero’ carbon UK buildings to become reality, engineers need to take an holistic view of projects and ensure they are making effective use of the energy source. John Dalley has long held the view that
applying integrated site-wide solutions, using proven technologies, will substantially reduce operating costs and provide truly ‘zero’ carbon site energy. A concept zero carbon (or ‘carbon neutral’) site powered by a centralised energy unit is shown schematically by Dalley in the figure below. The core of the installation is based around a two-stage ORC CHP plant. ORC technology has been successfully
applied in Europe since the 1980s and can be fuelled with a range of heat sources from biomass and refuse derived fuels (including processed sewage pellets) through to solar collectors, geothermal and ground sources. The ORC is suitable for waste heat recuperation, where a thermal oil transfers heat from the source to the ORC unit, and can use the heat rejected from other power processes such as reciprocating engines and potentially fuel cells. Dependent on the quality of heat input, the ORC unit would typically produce around 20% electrical output. As indicated in the figure below, the available heat resource can be almost wholly utilised through a combination of technologies
The principle of operation of a simplified ORC plant
Phase change heat store
STAT. STAT.
Thermal solar all-weather vacuum tube
that allow the capture, storage and delivery of heat. This will enable the highest exergy fraction to be maintained by consuming the heat where best use is made of its quality. The higher temperature heat is distributed
to (commercial) heating and, in conjunction with absorption refrigeration, to cooling loads. Low temperature heat may be used directly in applications such as underfloor heating, or potentially in conjunction with transcritical heat pumps to supply domestic hot water. The success of a carbon neutral site is dependent on having a balanced profile of loads to maintain maximum utilisation. The inclusion of thermal storage – in this case illustrated by a phase-change store employing eutectic salts but it could be ground or rock storage – will provide flexibility in matching available heat to the transient demand. To maximize heat benefit and to minimize pumping energy the temperature differences across the loads would preferably be more flexible, and greater, than the still widely adopted 80°C/60°C. Low exergy sources such as ground loops
are fed into the low grade ORC plant, while sources such as refuse-fuelled boilers would supply the high grade ORC plant. Linking the low and high grade ORC units could result in a combined electrical output of about 30%.
Linking the low and high grade ORC units could result in a combined electrical output of around 30%
JOHN DALLEY was formerly design manager at DIS and is now an independent consultant
Underfloor heating or radiators or fan coils
Heat recovery ventilation for house Filter
Distributed heating systems with controls and metering
Cooker hood
M
Cold water main
Thermal oil system
M M M M
High grade (ORC) plant
20%
Heating mains
Electrical distribution and metering
Two stage biomass/orc/ heating/electricity central generation plant
15%
Absorption refregeration
Indirect HWS
Heat dump
Central biomass or refuse derived fuel boilers
40
CIBSE Journal December 2012
www.cibsejournal.com
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