Introduction
Besides the project management the Swiss
contribution refers to both the development of a test procedure and a
calculation method. In Switzerland, a research focus
in the field of heat pumps is the development of so-called
“retrofit heat pumps” for the replacement of boilers in existing
buildings. These research activities led to the development of a
cascade heat pump using condensate subcooling for the domestic
hot water heating (awarded as “Swiss
retrofit heat pump” - SRHP). This system is now on the Swiss
market by the manufacturer
KWT.
As a first step, a test procedure and a
calculation method for this combined operating system with
cascade heat pump using condensate subcooling was developed. The
results were compared to an alternate operating Siemens heat
pump with DHW storage in parallel of the manufacturer
Novelan.
Test procedure
The test procedure, that was developed at
the former national
heat pump test centre WPZ Töss is based on the existing
standards EN 255-2 and EN 255-3, which cover the operation modes
"single heating" and "single DHW production". For the alternate
operating system these two standards are sufficient, since the
heating and domestic hot water operation is independent and thus
the two operation modes can be tested separately.
For the combined operation the heat pump
characteristic changes significantly and testing according to
these standards is not sufficient. Therefore the test procedure
provides another testing sequence performed in parallel, i.e.
the testing cycle for the domestic hot water according to EN
255-3 is accomplished during the operation of the heating
system.
Following the
black-box
approach, the electrical input to the system cannot be
redistributed to the two services of space heating and domestic
hot water production in simultaneous operation. In alternate
operating systems the heating operation is not influenced by the
domestic hot water. Thus the assumption is made that this is the
criterion for the redistribution as well in simultaneous
operation, i.e. a deterioration or amelioration of the COP-value
is entirely accounted to the domestic hot water operation. This
might not be physically true, but the approach has the
advantage, that the resulting COP-values are comparable to the
ones of alternate operating systems. For the calculation of the
seasonal performance only the total electrical energy is needed,
as only the overall efficiency in combined operation is of
importance. The results of the testing are the three
characteristics for the respective operation modes.
Calculation method
The calculation method is based on a bin
approach already used in the
NTH Project of the SFOE. The bin approach uses the
cumulative annual frequency of the heating degree hours vs. the
outside dry bulb air temperature. The total operation interval
is thereby divided into temperature classes called bins. The
ratio of the bin area to the total area reflects an energetic
weighting factor for the performance factor at this operation
point. This bin performance factor takes into account the energy
demand (for heating or DHW), the temperature conditions at the
respective operation point, systems losses for cycling operation
and eventually installed storages as well as additional
auxiliary energies not considered during testing. By this
energetic weighting of the bin performance factors the seasonal
performance for the whole operation interval can be calculated.
For the different services, e.g. heating
and domestic hot water, single seasonal performance factors are
weighted with the respective energy amounts for the service.
For simultaneous operation, the
calculation performs an energetic weighting with the seasonal
performance factor for the three operation modes "single
heating", "single domestic hot water", "combined heating and
domestic hot water" based on the component characteristic
delivered by the test procedure.
The respective energy amounts produced in
combined and single operation modes are evaluated based on the
running time, which is defined by the actual heat demand and the
output capacity at the respective operation point. As the
control system has an impact, as well, preliminary simulations
to deliver the fraction of combined operation have been
performed and expressed in a correction factor.
Perspective
In the second step this test procedure and
calculation method is to be validated and evaluated with regard
to the application to other system configurations. In
particular, this includes a further development to systems,
which contain ventilation, too, for instance compact units for
ultra low energy houses. Therefore field measurements of these
systems are planned in parallel to the extension of the testing
and calculation to handle air distribution system and combined
space heating/cooling, DHW and ventilation.
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