23.04.2014
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The essence of operation of such machine as heat pump is its work with heat transmitted during phase transitions. Therefore,
using special cooling media, we are able to receive heat at low air temperature values and transfer it to warmer water without
high energy expenditure. In some way, it is a system much like refrigerator - from its interior we carry heat out of cooling
chamber. In case of heat pumps the process is reverse, however analogical as regards its principle - heat is drawn from
environment and transferred inside utility water tank.
2
Savings
One of the main criteria for equipment purchase is to ensure savings during its operation. Especially, when we use electric
heaters or oil-fired boiler for HUW heating in summertime, we have economic grounds for switching to air heat pump. In case of
solid fuel fired boiler we speak less about economic profits compared to combusted fuel itself, because water heating costs are
comparable - here, decisive issues are: automatics of the system operation and possible costs, which have to be incurred for
chimney repair in case of boiler work at low parameters (flue gas condensation).
Below we show costs for 1kWh of heat in summertime for various automatic equipment:
1. For heat pump – average-season COP coefficient = 3.4; cost for
1kWh of electric energy 0.65 PLN
2. For natural gas assumed cost is 2.15 PLN/m
3
(fixed charges not
included), min. kWh content in 1 m
3
of gas (according to standard)
reaching 8.61 kWh/m
3
, actual efficiency of gas-fired boilers in
summertime (low-load work), near 70%
3. For oil heating, cost for 1l of oil 4.10 PLN, actual efficiency of oil-
fired boilers in summertime near 70%, kWh content in 1l of oil:
10 kWh/l
4. Cost of 1 kWh of electric energy, 0.65 PLN/kWh
In case of using heat pump for room cooling, return on investment will be even faster - cooling is a side-effect, and therefore it is
completely free and reduces return on investment period to 2-3 years.
3
Heat pump design
Main parameters of the compressor in heat pump system:
1) Increases cooling medium pressure and temperature to allow efficient
heat transfer in heat pump condenser
2) The compressor has considerable impact on the efficiency and
productivity of the whole process, because this heat pump section
consumes ca. 90% of electric energy.
Condenser. Shell-and-tube heat exchanger has been used as a device,
in which heat is transferred to utility water. Utility water flows through
copper tube with extended heat exchange area. Cooling medium flows in
jacket between housing and internal cylinder. During heat transfer it
condenses from gaseous form to liquid and returns process heat
previously drawn in the evaporator.
Due to the exchanger design, we can work
directly with utility water, considering both
hygiene
and
negligible
exchanger
susceptibility to scaling.
Rotary compressor has been used in the
PCWU 2.5kW heat pumps.
Medium state changes from liquid to
gaseous inside the evaporator.
During
evaporation, the medium draws a lot of heat
from air (this heat will be returned to utility
water during condensation). Due to low
specific heat of air, we have to force air flow
through evaporator using the fan.
Electronic,
automatically
controlled
precision expansion valve. Expansion valve
is expected to ensure that cooling medium
in evaporator draws heat from air washing
the evaporator. Heat transfer is generated
through medium evaporation. Depending
on medium temperature difference before
and after the evaporator, expansion valve
either opens (at the moment when high
volume of heat may be drawn in evaporator
- larger amount of the medium can be
evaporated) or closes (if too much medium
is being sent to the evaporator and all of it
cannot be evaporated because of
insufficient amount of heat in air). Expansion valve opening or closing
changes medium pressure before evaporator - and thus modifies medium
evaporation temperature (lower pressure - the medium boils at lower
temperatures, and for higher pressure - at higher temperatures).
Remember that soiled evaporator (e.g. with greasy air, pollens or leaves)
will draw heat from air in a considerably less efficient way.
electricity
heat pump
natural gas
fuel oil
Cost
