Global Warming Effects (updated 2010)
Vast amounts of heat energy are used for heating
our houses, and it is always advantageous to
minimise the heat requirements by normal conservation
measures like insulation. This is certainly
the best and simplest first-step. Sadly, we
cannot all live in super-insulated eco-houses,
so significant amounts of heat energy are required.
Any use of fossil fuel creates harmful CO2
emissions. Electricity is mostly produced by
burning fuels, and the process of generation
is inefficient. Electricity is therefore not
a good form of energy for heating. However,
the energy advantages of heat pumps can, in
most cases, more than compensate for this, and
makes them score well with respect to CO2
emissions.
Note: The assumption that you can sign up to
'green' electricity and become 'carbon neutral'
has not been made since electricity all comes
from the same 'pot'. However, it is clearly
an advantage to buy electricity from a genuine
supporter of lower-carbon generated electricity.
The nations methods of electricity generation
results in different levels of CO2 pollution.
Coal power stations are not good, modern gas
power stations are a bit better, hydro and large
scale wind power produce almost no CO2 (even
when all the energy to build them is accounted
for).
The 'mix' of generation leads to a figure that
is published at:-
Ecotricity
website and Realtime
Carbon. Its well worth a look.
The official average figure used for UK electricity
generation is currently being debated.The official
figure has been 0.422kg CO2 per kWh generated.
This allowed for some projection into a lower-carbon
future, and the current actual figure is on
average around 0.5. However, according to SAP
2009 version 9.90 the figure is now 0.591 kg
CO2/kWh. It appears however that the UK will
struggle to meet its predicted targets. No doubt
the figures will continue to be debated.
The graph compares the carbon dioxide emissions
from common heating systems (gas, oil and electricity)
to that of heat pump systems. The COP (efficiency)
for a typical Ground Source heat pump with radiators
is around 3, but if well designed underfloor
is used the efficiency can be 4. It could however
be as low as 2.5 for a poorly designed system.
I have taken a figure for electricity as 0.5
which is our UK presentt actual average. It
is also mid way between the old and the new
value.
Notes and assumptions.
Gas condensing boiler efficiency 90%
Oil boiler efficiency 87%
LPG condensing boiler efficiency 89%
Wood varies from 0 to 0.06 depending on processing,
transportation and burner efficiency.
Electricity 100% efficient at point of use
Note: Electricity is most polluting during peak
times when older coal generators are brought
on-stream.
As can be seen, the carbon dioxide contribution
from a good heat pump system can be 1/2 that
of oil or LPG. Direct electric heating, especially
storage heaters, scores very badly with respect
to global warming.
Wood burning is likely to show the lowest CO2
figures but this depends on how it is burnt,
and where the wood is sourced from. Locally
collected logs are carbon neutral, but processed
pellets and chips require energy to make. Currently
some wood fuels are imported.
Many heat pumps incorporate a normal electric
top-up heater to cope with the coldest periods
in the winter. This is more the case with air-source
systems since the period of highest heat-demand
also corresponds with the time when there is
minimum heat available in the air. Therefore,
on the coldest day the electricity consumption
for some heat pumps will increase many fold,
putting a strain on the electricity supply grid.
It would be better to use boilers, or ideally,
wood stoves as a back-up. It should however
be notes that the total annual contribution
by top-up heaters is surprisingly small.
The COP (coefficient of performance) of a heat
pump is the ratio of input to output.
Fuel figures SAP 2009 version 9.90 / Defra.
============================================================
The Graph below might
be useful when assessing the benefit of running
a heat pump. (its not
as complicated as it looks!)
The vertical scale on the left shows CO2
pollution per kWh of heat delivered. The two
horizontal lines show figures for mains gas
and oil heating. The curves are for heat pumps
with COP's up to 5. Direct electric heating
has a COP of 1, and this is shown on the left.
The UK government figure for CO2 pollution caused
by electricity generation has been 0.422 (kg
co2/kWh). However SAP 2009 lists it at 0.591
(kg co2/kWh). The actial present figure is around
0.5, and will tend to be high in mid winter
when coal power station are brought on-line
to meet increased electricity demand.
Given current figures of 0.5, it is necessary
to have a COP of about 1.7 to equal the pollution
caused by oil heating.
You can use this graph to decide if its better
to run your heat pump or your gas boiler depending
on your systems COP, and the figures for mains
electricity.
The blue line represents the future when the
electric grid becomes de-carbonised and circumstances
are good for heat pumps
Click below for some amazingly useful
tools that gives actual UK generation CO2
figure
Real
time carbon
Ecotricity
===============================================
REFRIGERANTS (the working
fluid)
Gone are the days of refrigerants affecting
the ozone layer. They are now all 'ozone-friendly'.
However, most refrigerants do have a high global
warming potential, meaning that they are far
more potent than the gas - CO2.
The refrigerant fluid within the heat pump
is very unlikely to leak (when did you last
hear of a refrigerator leaking its refrigerant?)
but should it do so, it could detract from the
CO2 savings. The
following information attempts to give a balanced
view of the issue and concludes that the savings
made during the heat pump's working life, far
outweigh any negative effects caused by loss
of refrigerant.
The following table lists the Global Warming
Potential (GWP) of various gases. This gives
a figure of how damaging the gas is, as compared
to CO2.
| Refrigerant |
Global Warming Potential
|
| Carbon Dioxide, CO2
|
1 |
| Hydrocarbon, HC |
7 |
| HFC R407C, R410A, R134A |
1,900 |
Inside a typical 8kW heat pump there is approximately
2kg of HFC refrigerant. This should stay intact
within the unit for its whole life. It is a
legal requirement to recover the refrigerant
charge at the end of the heat pumps life. This
can then be used in another heat pump, or destroyed.
In this worse case scenario, we have assumed
that the refrigerant is lost via a leak during
a 10 year period.
The first five columns show the total CO2
released to atmosphere over the 10 years of
running each system. The saving in CO2
that you would make if you have a heat pump
would be the difference between the heat pump
column (purple), and the fuel it replaces to
the left.
The common HFC refrigerants (shown in pink)
have a relatively small effect compared to the
vast amount of CO2
produced by using energy over a long period.
Like your domestic fridge, a heat pump is unlikely
to lose its refrigerant charge over its life,
so this column could prove to be very small
in reality.
Graph taken from expected consumption for a
year 2002 regs. house.
10,500kWh space heating, 3,600kWh. hot water.
Whilst this might show that HFC refrigerants
are acceptable, it is obviously preferable to
use an Hydrocarbon (HC) refrigerant if possible.
If heat pumps become widespread, then benign
refrigerants like hydrocarbons should ideally
be adopted.
We have added wood to the graph to indicate
how good logs are as a back-up for a system.
Notes and assumptions
Gas condensing boiler efficiency 88%
Oil boiler efficiency 85%
Electricity 100% efficient at point of use
Heat Pump COP 3.8 (i.e. 2.1 kW input for 8kW
output).
