Efficiency
The word "Efficiency" is defined as
the ratio of useful heat output to energy input.
e.g. if an open fireplace loses half its energy
up the chimney it is said to be 50% efficient.
Efficiency is commonly used to describe how
effective something is. On this website the
term efficiency relates to energy efficiency.
COP
The COP or 'Coefficient of performance' is found
by dividing the useful heat output by the energy
input. e.g. a heat pump that produces 4 kWatts
of heat for 1 kWatt of input power has a COP
of 4. The open fireplace example with 50% efficiency
would have a COP of 0.5. (1/2)
SPF
Seasonal Performance Factor is similar to COP,
but is an average figure taken over the year.
It is usually lower than quoted COP figures,
especially if back-up electric heaters are used.
Source
This is wherever the heat is being extracted
from. eg. the outside air, river or ground.
Sometimes referred to as an ambient source.
Spring. This is where water
comes directly from the ground.
Stream, a small river.
Sink
This is the name given to the part where the
heat is usefully dissipated, such as radiators
in the room, underfloor heating etc.
Emitters
Another term used to describe radiators or underfloor
heating. This is the component that emits the
heat into the building.
Open Loop
This is the type of source where river or ground
water is pumped through a heat pump then expelled
to the environment a few degrees colder.
Closed loop
This is where a sealed plastic ground pipes
are used which usually contains a glycol antifreeze.
i.e. the most common trench or borehole system.
DX system
Abbreviation for 'direct expansion'. This is
where the refrigerant flows directly within
the ground pipes. This system is less common,
and may have some disadvantages, however, it
can promise higher efficiencies since there
is one less pump and one less heat-exchanger.
Slinky
The name sometimes used to describe the type
of ground collector pipes which are coiled before
burying in a trench.
Horizontal collector
This can be either coiled 'Slinky' or straight
pipes that are buried up to 2m deep in open
ground (your garden). The pipe is usually plastic
and contains a Glycol antifreeze solution.
Borehole
This is simply a vertical hole drilled
in the ground. A ground source collector pipe
can be installed in this.
Antifreeze
This is simply an additive that gives water
a lower freezing point. Ethylene or Propylene
Glycol is most commonly used in heat pump systems.
Brine
Brine is normally defined as 'salt water'. However,
this term seems to be have adopted to describe
any antifreeze mixture. A brine-water heat pump
usually means one having glycol antifreeze on
the 'cold' side and water on the 'hot' side.
Refrigerant
This is the working fluid within the heat pump.
It evaporates in one part and condenses in another.
By doing so, heat is transferred from cold to
hot. This fluid is sealed in and will not degrade
within the life of the heat pump.
Heat Exchanger
This is a simple component that transfers heat
from one fluid to another. It could be liquid
to liquid, liquid to air, air to air. Two heat
exchangers are housed within the heat pump,
one for the hot side (the condenser), the other
for the cold side (the evaporator).
De-superheater. This is a
small heat exchanger fitted to the compressor
discharge that can produce a small amount of
heat at a higher temperature.
Passive Cooling.
Passive cooling is where the ground water is
simply pumped around underfloor heating. This
gives limited amount of free cooling. It
will only work with boreholes or large trenches
in very wet ground. We repeat:- a
limited effect, but its free!
Passive
heat recovery ventilation
This is where the out-going exhaust air passes
its heat to the incoming fresh air with only
the use of a simple heat-exchager. It uses no
heat pump.
Geothermal
This is defined as 'heat from the ground'. Proper
geothermal is heat from the earth's core extracted
from very deep in the ground, as in the steam
that powers the whole of Iceland. The term seems
to have been adopted to describe heat pumps.
We prefer the use of the term 'Ground source
heat pump' .
Buffer tank
This is simply a large water cylinder that is
used to improve the efficiency and durability
of a system. It reduces the number of stop/starts
that the compressor makes, and ensures a high
flowrate through the heat pump.
Heat Pump Rating.
A heat pump is given a kW heat output
rating. This value will vary depending on the
working temperatures.The electrical power input
will be between a 1/2 and a 1/4 of the heat
output.
Inverter. This is a sophisticated
electrical device that can vary the capacity
of a heat pump. It therefore can vary the heat
output to match the heat demand. (An inverter
is also a soft-start)
Soft Start. This is an electrical
device that reduces the start surge that is
taken by a conventional compressor. It does
not save energy, but stops lights flickering,
and may reduce wear & tear.
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Some useful figures and conversions.
1 kW (kilowatt) is a unit of Power or a rate
of energy. (A 1 bar fire consumes 1 kW)
There are 3,411 Btu's in 1 kWatt. i.e.10kWatts
= 31,400 Btu/hr.
There are 860 kcal/hr in 1 kW
A normal immersion heater uses 3kW when heating
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1 kWh. (kilowatt hour) is a quantity of energy
( A 1 kW heater would use 24kWhr per day )
I kWatt Hr. = 1 unit of electricity = 1 bar
fire used for one hour.
Note gas bills now use kWhr. instead of the
old confusing units. Therms etc.
1 kJoule x 3,600 = 1 kWhr.
Note heat pumps are usually rated by their
heat output, not their electricity input.
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If 10Kwatts were extracted from water having
a flow rate of 0.8Lit/sec then the temperature
would drop by 3°C (3K).
A heat pump with a heat output of 10kW and
a COP of 4 can be represented by the following
equations:-
COP = heat output / electricity consumption
4 = 10kW / 2.5kW
the heat extracted from the ground = heat delivered
– electricity consumption = 10kW –
2.5kW = 7.5kW
0°C = 32°F (freezing point of water)
10°C = 50°F
20°C = 68°F (room temperature)
100°C = 212°F (boiling point of water)
or, if you have a calculator, °F-32,/9,x5=°C,
°Cx9,/5,+32=°F
1 lit/sec = 3.6m³/Hr. = 13.19 Galls(UK)/min.
This chart gives an indication of flow rates
for a 10kw (heat output) heat pump taking heat
from a river or spring source.
(the extracted heat has been assumed to be 7.5kW)
| Source inlet temperature |
Source outlet (return) temperature |
Flow rate Lit/sec. |
Flow rate
M³/h. |
Flow rate Gallons/ min. |
| 6°C |
3°C |
0.6 |
2 |
28 |
| 10°C |
7°C |
0.6 |
2 |
28 |
| 10°C |
4°C |
0.3 |
1 |
14 |
For a river source system the flow rate would
need to be at least 0.6 liters/ second to avoid
freezing in the evaporator.
For a spring source, the flow rate would ideally
be that same, but if the supply was limited,
then half that rate may suffice.
Note, these are example values. Manufacturers
data should be available for specific equipment.
Spring water source purity figures.
Water purity for normal copper-brazed stainless
heat-exchangers as used in almost all heat pumps.
The following list will give some idea of the
requirements. Check with the heat pump manufacturer
to get specific data relating individual heat
pumps.
Sulphate < 100 mg/l
Free chlorine < 0.5 mg/l
Chloride < 300 mg/l
Nitrate < 100 mg/l
pH value 6.5 - 9
Electr. conductivity 50 - 1000 µS/cm
Oxygen < 2 mg/l
