Note: Descriptions are shown in the official language in which they were submitted.
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AIR-SOURCE HEAT PUMP
The present invention relates to air-source heat pumps, which extract heat
from air at ambient temperature. Heat rejected at a higher temperature is used
as a
heat source for heating buildings etc.
The concept of the heat pump has been known for many years. It operates by
extracting heat that is available from a source at a particular (lower)
temperature and
rejecting the heat together with the energy required to drive the heat pump at
a higher
temperature. The rejected heat is typically used to heat a living space, such
as a
building, and may heat the air in the living space directly or via the use of
a
recirculating heat transfer fluid, such as water. A refrigerator is a type of
heat pump
in which the main objective is to remove heat from the object being cooled. In
some
circumstances, it is possible to make use of the heat rejected as well as
benefiting
from the refrigeration effect. Such heat pumps are particularly economic.
However,
in most circumstances it is difficult to match heating and cooling
requirements; and in
practice most systems are used either for heating or cooling.
For many years, heat pumps were not generally economic because fuel prices
were low and there was little concern about emission of carbon dioxide from
the
burning of fossil fuel. The situation has now changed because fuel prices are
rising
and there is serious concern about the effects of increased concentrations of
carbon
dioxide in the atniosphere.
Heat pumps operating in a vapour compression cycle require a source of
energy to undertake the compression stage. In an absorption type heat pump,
dissolved refrigerant and solvent is pumped to high pressure as liquid and
heat is used
to boil refrigerant out of the solvent at a pressure under which the pure
refrigerant can
be condensed. The pure refrigerant is then reduced in pressure and transferred
to an
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evaporator where it is capable of extracting heat. In an alternative
arrangement, an
electrically or mechanically driven compressor is used, thereby avoiding the
burning
of fossil fuel in the building. In vapour compression heat pumps, the amount
of heat
energy rejected into the building is normally several times greater than the
amount of
energy used to drive the heat pump. The use of heat pumps for the heating of
buildings is now becoming economic and may, in some circumstances, become
mandatory. A large proportion of the heat output from a heat pump is provided
by the
environment and is therefore renewable.
Heat pumps are known which extract heat from the ground, or from ground
water. However, the present invention is concerned with heat pumps which use
ambient air as the heat source. In an air-source heat pump, heat is extracted
from
ambient air, which heats evaporating refrigerant in the evaporator. Work is
then done
on the refrigerant to compress it and heat it to a higher temperature. Heat is
output
from the compressed refrigerant at a higher temperature and is used as a heat
source
for heating the building etc. Generally speaking, heat pumps are at their most
efficient when the difference between the temperature at which heat is
absorbed and
the temperature at which heat is rejected is small. However, this is the
situation in
which there is least requirement for heating of the building. The greatest
quantity of
heat is normally required when the ambient air is at its coldest and the heat
pump is
therefore at its least efficient.
Another disadvantage is that if the temperature of the evaporator is allowed
to
become too low due to the use of particularly cold ambient air, there is a
danger that
the evaporator will become choked with condensed water vapour in the form of
frost.
Thus, it is a disadvantage of air source heat pumps that some source of
supplementary
heating is normally required in cold weather. For example, it is known to
provide a
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source of auxiliary heating, but this is usually done electrically. When the
ambient
temperature is particularly low, an auxiliary heater can be activated to
provide
additional heating, usually directly into the building to augment the heat
from the heat
pump. As the ambient temperature becomes colder, the efficiency of the heat
pump is
reduced and the need for additional auxiliary heating increases.
However, it is known from patent US4191023 to provide an auxiliary
combustion heater, which operates as part of the heat pump itself. In this
case, the
auxiliary heater boils refrigerant in a secondary circuit before it reaches
the
condenser, and increases the amount of heat rejected from the heat pump.
However,
such system does not address the problem of frosting in the evaporator.
The present invention seeks to mitigate these disadvantages.
Broadly speaking, the present invention relates to the use of a heater which
directly heats the incoming air to an air-source heat pump and thereby
prevents
frosting when the ambient air temperature is low. This effectively increases
the
temperature of the ambient air and thereby increases the efficiency of the
heat pump,
as well as reducing frosting.
Specifically, the present invention provides an air-source heat pump wherein
heat is extracted from air at ambient temperature by an evaporator and heat is
rejected
at a higher temperature in a condenser; the heat pump further comprising an
auxiliary
heater arranged to preheat the ambient air when required to prevent deposition
of frost
on the evaporator.
Generally speaking, the auxiliary heater will only be operated when there is a
risk of frost forming on the evaporator surfaces, for example under conditions
of low
temperature. Generally speaking, the auxiliary heater will'be arranged to
provide
auxiliary heat to the ambient air when the ambient temperature falls below 10
C,
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particularly below 5 C, especially below 2 C and most especially below 1 C.
When
the relative humidity of the ambient air is high, the heating effect of the
ambient air is
greater and there is therefore less need for auxiliary heating.
The auxiliary heater will usually be arranged to operate by monitoring the
evaporating pressure (or temperature) of refrigerant and then applying
sufficient heat
by means of the auxiliary heater to prevent the refrigerant evaporating at a
pressure
that would allow frost to form. Alternatively, the auxiliary heater could be
controlled
dependent on the temperature and humidity of the ambient air; but this is more
difficult to do in practice.
The auxiliary heater may be an electrical heater or may use waste heat from
other sources. However, it is particularly preferred to employ a combustion
heater,
wherein flue gas from the combustion heater mixes with the ambient air. The
ambient
air itself may be used as the air source for the combustion heater or the air
may come
from elsewhere. Although the combustion heater itself contributes to the
carbon
dioxide emission of the system, it is nevertheless a relatively cheap and
efficient form
of auxiliary heating and one that will not be required during most of the
heating
season in a temperate climate. The conditions will generally be arranged such
that
water as a product of combustion within the flue gases condenses as liquid on
the
surfaces of the evaporator and will thereby allow the latent heat of
evaporation to be
recovered. The auxiliary combustion heater generally uses fossil fuels, such
as
hydrocarbons as the fuel source. It has been found that gas (such as North Sea
gas),
which has a low sulphur content does not cause corrosion of the evaporator
surfaces.
The hydrocarbons are generally liquid or gaseous hydrocarbons, more
particularly
with less than 10 carbon atoms. C1-C6 hydrocarbons are preferred, especially
natural
gas (which is largely methane), propane or butane.
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In a preferred embodiment, the auxiliary heater is a combustion heater (e.g. a
gas-fired boiler) which heats a fluid used to provide heat directly to the
building (or
anything else to be heated). Typically, the fluid is air or recirculating
water. The
heater is operated when additional heating is required, and provides heat
directly to
the building via the heated fluid. Also, flue gases from the boiler are mixed
with
ambient air before the air is passed over the evaporator, and thus provide
additional
heat input to the heat pump.
The gas fired boiler may be of the condensing type, to improve efficiency and
reduce the heat content of the flue gases relative to the heat content of the
heated
fluid. It may alternatively be of the non-condensing type.
The heated fluid is usually air or recirculating water. Other gases or liquids
could be used for other applications. Recirculating water is generally used to
heat
radiators. The heated air or water may provide under-floor heating in a
building.
The heat pump may employ any suitable refrigerant. A particular benefit of
the present invention is that it allows the temperature at which heat is
extracted (i.e.
the evaporating temperature ) to be controlled so as to improve the
performance of the
heat pump. The heat pump typically operates in an evaporation-condensation
cycle
using a source of energy to compress the volatile refrigerant. The source of
energy
may be a compressor (such as an electrically driven compressor) or may be a
liquid
pump plus a source of heat in an absorption type system.
Typically, the heat pump will be arranged to extract heat from ambient air
(preheated if necessary) in the range 5 - 15 C, particularly 7 - 12 C. Heat is
typically
rejected at a temperature in the range 30 - 35 C for direct air heating or for
under-
floor heating and, typically 50 - 55 C for the heating of circulating water.
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An embodiment of the present invention will now be described by way of
example only, with reference to the attached figure, wherein
Figure 1 is a schematic drawing of an air-source heat pump according to a
first embodiment of the present invention, which utilises a combustion heater
as the
auxiliary heater; and
Figures 2a and 2b are schematic drawings of a second embodiment, where the
auxiliary heater is a gas-boiler which also heats a flow of recirculating
heating water.
Briefly, Figure 1 shows the circuit of an air-source heat pump having a
compressor 1, a condenser 2, an expansion device 3, an evaporator 4, a casing
5, a fan
6, a gas burner 7, and interconnecting piping 14. The heat pump contains a
volatile
refrigerant which is re-circulated.
In operation, liquified refrigerant from the condenser 2 at a temperature
55 C is expanded through an expansion device 3 into an evaporator 4 wherein
the
refrigerant evaporates within the pipes of the evaporator and extracts heat
from
ambient air. The refrigerant at a temperature of 5 C is drawn from the
evaporator by
the compressor 1, is compressed again to condensing pressure and is condensed
in the
condenser 2. Condensing refrigerant at a temperature of 55 C rejects heat to
the
building to be heated and is thereby liquefied at a temperature of 55 C. The
heat
extracted from the air stream plus the equivalent of the work put into the
compressor
is rejected from the condenser. The refrigerant then flows through the
interconnecting
piping 14 back to the expansion device 3. At the expansion device the
refrigerant is
reduced to evaporating pressure and returned to the evaporator, where the
cycle
recommences.
Ambient air 10 is the source of heat for the heat pump. Ambient air is drawn
into the casing 5 by means of a fan 6 and exits in the direction of the arrow.
Under
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normal circumstances where the temperature of the ambient air is not too low,
heat is
extracted from the ambient air by heat exchange with the surfaces of the
evaporator 4.
However, when the temperature of the ambient air reduces there is the danger
of frost
collecting on the evaporator surfaces. The temperature and/or pressure of the
evaporator may be monitored by a sensor means (not shown) and when a
predetermined value is reached, the combustion heater 7 is operated. The
combustion
heater uses natural gas (which generally speaking does not produce corrosive
flue
gases which might damage the evaporator surfaces). The auxiliary combustion
heater
7 is operated to bring the temperature of the ambient air back to a preset
temperature,
whereby the danger of frosting is avoided and the efficiency of the heat pump
is
improved. Water that condenses on the evaporator surfaces may be arranged to
drain
away via a drain (not shown).
Thus, under normal operating conditions, heat is extracted from ambient air
without producing frost on the evaporator surfaces. However, when the ambient
air
temperature falls to such an extent that frost would otherwise form on the
evaporator,
the auxiliary gas burner 7 is ignited, thus providing an additional source of
heat and
preventing the formation of frost. The burner may be controlled to provide a
preset
temperature in the ambient air flowing over the evaporator surfaces, or may be
controlled to prevent the refrigerant evaporating at a temperature below the
freezing
point of water.
Example
Using a selected compressor evaporator and condenser utilising ammonia as the
refrigerant, it can be shown from manufacturer's data that with an air at a
temperature
of 15 C (75% relative humidity), and an air flow of 17 cubic m/s, the heat
extraction
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will be 140 kW when the refrigerant evaporates at 10 C (6.15 bar absolute) and
condenses at 55 C. Under these conditions the compressor will absorb 34.7 kW.
The coefficient of performance (heating) is therefore (34.7 + 140) / 37.4 =
5.02.
The limit of operation of the system before frost is formed is when the
refrigerant evaporates at 0 C (4.19 bar absolute). When refrigerant evaporates
at
0 C and condenses at 55 C the refrigerating system will extract 101.7 kW of
heat and
absorb 33.7 kW of power. The coefficient of perfonnance (heating) is therefore
(33.7 + 101.7)/ 33.7 = 4.02.
Thus even under limiting conditions the heat pump will still produce more than
four
times the input energy in the form of heat.
It can be calculated that the ambient temperature is about 3 C at the limiting
condition
when the refrigerant evaporates at 0 C. If ambient temperature falls below 3
C, the
auxiliary heater will be engaged progressively till all the heat is being
provided by
the auxiliary heater.
In a temperate climate such as that of Great Britain or Japan, the temperature
rarely
falls below 3 C for any considerable period of time. The invention described
therefore provides an effective method of providing heat without having to use
fossil
fuel except in exceptional circumstances. When fossil fuel does have to be
used, it is
used at maximum efficiency because the products of combustion are cooled down
to
about 3 C.
Figures 2a and 2b show a second embodiment (analogous parts are marked
with the same reference numerals as in Figure 1) where the gas burner 7 not
only
provides flue gas into the evaporator 4 but also heats a recirculating flow 15
of heated
water, which is used for heating the building (together with heat from the
condenser
2).
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Figure 2a shows a gas-fired boiler 7 housed in a separate chamber defined by a
partition 8. Air flow 9 supplies the boiler with combustion air and flue gases
12 exit
via boiler flue 13. Ambient air 10 enters the heat pump unit through louvres
16 in the
wall of the casing 5, and mixes with flue gases 12 from the gas boiler. The
mixed gas
stream then passes over evaporator 4, as described above, and exits as stream
11.
A heated water circuit 15 is provided to heat radiators etc., within the
building.
It picks up heat rejected in condensor 2 during normal operation of the heat
pump.
However, when additional heating is required the gas-fired boiler 7 is
operated. This
provides additional heat to the recirculating water 15 and also provides flue
gases,
which provide additional heat to the ambient air 10 flowing over the
evaporator 4.
This provides a balance of direct heating via water stream 15 and indirect
heating via
the heat pump, which can be optimised to suit different conditions.
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