COMBINED HEAT AND POWER 1 ENERGY EFFICIENCIES
Variation of heat emissions factor for various heat sources 300% 250% 200% 150% 100% 50% 0%
300 350 400 450 500 550 600 650 700
-50% -100% -150%
Steam turbine extraction z=7
CHP at 42% CHP at 35% CHP at 25% Gas boiler at 85% Heat pump (CoP 3) Heat pump (CoP 2)
from the network. In these circumstances, CHP can supply 80% of annual heat consumption, with the remainder coming from boilers. The growing use of larger-scale CHP as part of DH systems has implications for the siting of power stations in the longer term – they will need to be nearer major cities. Larger heating networks will also need to be provided to take advantage of heat from such power stations. This will mean, in effect, that all new coal or
CO2 emissions per kWh electricity (g/kWh)
Figure 2: CO2 emissions for heat from various sources against electricity emissions factor. CHP efficiencies in the key relate to electrical efficiency; total CHP efficiency (thermal plus electrical) is kept constant at 80%. All efficiencies are on GCV basis
is equivalent to a heat pump with a CoP of 3.
l Below an electricity emission factor of about 420g/kWh, the lowest CO2 content is obtained by extraction of heat from an established steam turbine power station, which is equivalent to a heat pump with a CoP of 7.
What about district heating? The greatest benefit of CHP will come through the use of district heating, which will enable larger-scale CHP to be used, including using CHP to extract heat from power stations. DH also has energy inputs to counteract heat losses
gas power stations and biomass/waste plants will need to be ‘CHP ready’ to be able to supply heat in the longer term. DH will also be able to use a range of other heat sources, including large-scale heat pumps, solar, deep geothermal, electrode boilers and offer important demand- side management benefits, enabling more wind energy generation and reducing the need for electricity storage. In conclusion, it has been shown that, as the electricity grid decarbonises in the future, gas- fired CHP systems will save less CO2. CHP and DH schemes will need to evolve to maintain an environmental and economic benefit over other low carbon solutions, including taking heat from major low carbon thermal power stations and the use of large-scale heat pumps and thermal storage.
l Paul Woods is technical director of AECOM.
www.aecom.com This article is extracted from a paper presented to the CIBSE Technical Symposium 2011.
Calculating the energy efficiency of CHP and heat pumps useful heat output (Hchp
The energy efficiency of heat supply can be calculated using the ‘equivalent heat efficiency’ concept and used to compare heating options. For a boiler, equivalent heat efficiency = heat output / fuel used (equation 1) For a heat pump, equivalent heat efficiency = heat output/primary energy input = heat output / (electricity used / grid efficiency) = CoP x ηe,grid
(2)
Where: CoP = useful heat output / electricity consumption over the year ηe,grid
= grid efficiency = delivered
electricity / primary fuel input (Fps
) (3)
For a CHP system the performance requires the definition of two efficiencies*: The thermal efficiency = ηh
= 32 CIBSE Journal December 2011 ) / fuel input (Fchp) (4)
The electrical efficiency = ηh electricity generated (Echp) / fuel input (Fchp) (5)
=
The equivalent heat efficiency for CHP = heat output (Hchp
) / net
fuel used where: net fuel used = CHP fuel (Fchp
) less power
station fuel (Fps) displaced by CHP electricity generated (Echp
/ (Fchp – Fps ) (6)
From (3) above = Hchp / (Fchp (Echp / ηe,grid
))
From (5) above = Hchp / (Fchp ((Fchp x ηe
) / ηe, grid )) Dividing through by Fchp leads to
the following equation: CHP equivalent heat efficiency ηh,eq
ηe,grid
= (Hchp / Fchp) / (1 – (ηe ))
From (4) ηh,eq = ηh ηe,grid
) (7) / (1 – (ηe / / – – )
So equivalent heat efficiency, ηh,eq = Hchp
Or, in words: CHP equivalent heat efficiency = CHP thermal efficiency / (1 – (CHP electrical efficiency / grid efficiency) (7) Equation 7 is fundamental
to CHP performance. One implication of the second law of thermodynamics is that the highest electrical efficiency will be achieved when discharging heat at the lowest possible cold sink temperature (the ambient temperature). For heat to be useful, it will
always need to be above ambient, and so the CHP electrical efficiency will always be lower than the best grid efficiency**. In equation 7, as the CHP electrical efficiency tends to the grid efficiency, then the equivalent heat efficiency tends towards infinity – that is, the heat would eventually
be rejected close to ambient temperature and could be classed as ‘waste heat’. At the other extreme, if the CHP electrical efficiency tends to zero, then the CHP equivalent heat efficiency tends towards the CHP thermal efficiency; in other words, the CHP is tending to become the same as a boiler. Therefore the most efficient CHP is one where the electrical efficiency is as high as possible.
*The CHP performance can also be defined using: total efficiency = (heat plus electricity) divided by fuel input, or heat to power ratio plus one of the two efficiencies defined here, but two parameters are always needed to define CHP performance.
**Electricity grid losses also need to be considered where power stations are remote from, and CHP plant is close to, the point of consumption.
www.cibsejournal.com
CO2
emissions per kWh heat (g/kWh)
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