PUBLICATION DATE: Q1 2009
REF NUMBER: Refrigeration SWG 03
03
REFRIGERATION SPECIAL WORKING GROUP
Recovering Heat from
Refrigeration Systems
Cooling systems offer ‘free heat’
All cooling applications offer opportunities to recover heat. Refrigeration systems are no exception.
All refrigeration systems act as heat pumps, moving heat from a cold process (evaporator) to a condenser, from
where it is rejected into the atmosphere. The cooling may be for the process cooling, air conditioning or other
use. Thus all the heat removed from the process, plus most of the energy added by the compressor and ancillary
pumps, is rejected to the local environment.
This rejected heat can often be economically recovered and be used instead of heat generated from fossil fuels.
While there is a cost in running the refrigeration plant, this heat is effectively close to ‘free’.
What are the advantages?
•Given the right application and design, the payback can be rapid.
•A significant amount of heat can often be recovered.
•It may be possible to reduce the condensing temperature and
electricity consumption of the refrigeration compressor.
•If the chiller has been designed with direct refrigerant injection,
recovering the heat through external oil cooling will increase the
compressor capacity.
Uses for heat
•Low-grade heat for underfloor heating.
•Sanitary hot water.
•Preheating air or processes.
•Space heating in winter.
•Reheat coils for humidity control.
•Preheating water for washing purposes.
•Preheating boiler feedwater.
What are the disadvantages?
•In some cases, running at a higher condensing pressure may
involve a cost penalty (this depends on the relative costs of
heating and of electricity, and needs to be assessed on each site).
Technical assessment
Financial assessment
Recovered heat replaces hot water or steam, which is usually heated
with a fossil fuel, whereas input energy to the refrigeration system is
usually electricity. The cost of electricity is typically about three times
that of heat.
Source of heat:
source
temperature levels
Checklist
Condenser
30 to 45°C
Superheat in the discharge gas
70 to 110°C
Oil coolers
over 55°C
• Optimise the operation of the refrigeration plant. In doing this,
minimise all discharges of heat from the plant. This includes
optimising the use and distribution of cooling fluids as well as the
refrigeration machinery itself.
• Optimise the use of the fluid to be heated, i.e. minimise the heat
required. Do not waste ‘free’ heat.
• Assess opportunities to use the recovered heat. This needs to
include required temperature, required flow, time of usage and
distance from heat source.
• A heat-recovery project works better if a constant heat load all year
round is needed on the site.
Heat Recovery
Investigation
notes
tools / techniques
Optimise the use,
distribution and operation
of refrigeration plant
Ensure that the plant is already operating
efficiently and that the waste heat really is waste
and not the result of inefficiencies.
1. Minimise use of refrigerated fluids.
2. Minimise heat input into refrigerated
systems from system auxiliaries such as
distribution pumps.
Examine waste
heat streams in
refrigeration plant(s)
Quantify heat being rejected and verify against
system COP balance. Include quantity of heat, its
temperature and timing of its availability.
1. Measure air or water flow and temperature
difference to calculate heat content.
Quantify potential value
of this heat
Convert energy content to financial value based
on the cost of thermal energy. This will give an
indication of the potential for a heat recovery
project.
1. Validate against total system electrical
load and COP.
Do the financial savings look attractive?
1. Take estimated future costs of energy
into account.
2. Don’t forget to include CO2 emissions and their
value if part of EU ETS.
Examine uses for the
waste heat
Do you have a need for heat close to waste heat
source? Do the timings match (heat storage
may help but reduces economic attractiveness)?
Ensure heating systems are running efficiently.
1. Potential uses: boiler water make-up heating,
boiler combustion air pre-heating, HVAC fresh
air pre-heating (cold weather only), wash
water, others.
Quantify useable heat
and it’s value
Quantify the amount of available heat that you
will actually be able to use and put a financial
value on it.
1. If heat source is not hot enough to heat
to final temperature could it be used as a
preheat source?
No
Interesting?
Interesting?
No
End
End
Design and cost the necessary modifications to
recover heat including heat exchangers, pumps,
fans, pipes, ducts, wiring, controls as appropriate.
Design heat
recovery system
financial
appraisal
Do the financial savings look attractive?
No
End
Carry out a financial appraisal to compare the
potential savings with costs and decide if they
meet the organisation’s investment criteria.
Develop and
implement project
Sustainable Energy Ireland
Glasnevin, Dublin 9, Ireland
Glas Naíon, Baile Átha Cliath 9, Éireann
T. +353 1 8082100
F. +353 1 8372848
info@sei.ie
www.sei.ie
Sustainable Energy Ireland is funded by the Irish Government
under the National Development Plan with programmes
part financed by the European Union.
1. Simple payback, NPV, IRR