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As a hydronics expert, Grundfos supplies variable speed pumps and smart
sensors for the latter and has already enabled superior energy savings for
global heat pump manufacturers. Furthermore, a flow controlled system can be
used for heat metering, which is the focus for the following article.
In the
first part of this article, the accuracies of the Grundfos Flow Sensor are
listed and compared with those requested in the Energy Saving Trust (see in details http://www.energysavingtrust.org.uk/) RHPP, where a total maximum error
on the SPF value of 10% (on a yearly basis) is allowed.
Calculating
the SPF value, 4 sub-assemblies are required: a calculator (or display system),
a temperature sensor pair, a flow sensor and an electrical meter.
The total
error is calculated as the arithmetic sum of the maximum permissible errors of
each sub-assembly:
SPFError= Ec + Et + Ef + Ee < 10%
Ec =
Maximum permissible error (MPE) calculator
Et =
MPE temperature sensor pair
Ef =
MPE flow sensor
Ee = MPE electrical meter
A Grundfos
Flow Sensor is delivered with a maximum error of 1.5% of the full scale value
as a standard for heat pump applications. As the error is a percentage of the
full scale value it is necessary to consider closely which sensor range to
utilize. Thus, it is clear that a 2−40 l/min range will perform
better at lower flows, compared to a 5−100 l/min range.
Grundfos
Direct Sensors™ has been directly involved in a project with the Danish
transmission system operator (Energinet.dk) which included heat metering in 300
residential housing for a period of 1−2 years. The application scope of
the project indicated that either the VFS (Vortex Flow sensor Standard) 5−100 l/min
or VFS 2−40 l/min sensor would be the preferred range. The 5−100 l/min
benefits from a lower pressure drop, while the 2−40 benefits from a
higher accuracy - when no flow rates above 40 l/min are likely.
Experience
gained by Grundfos from the first 75 installations, indicated that no flows
higher than 33 l/min in the heating systems were detected. Due to the
better accuracy provided, it was decided in this project to use the VFS 2−40 l/min
for the remaining 225 installations.
Please see
the diagram below showing accuracy of a standard VFS 2−40 l/min
sensor at an independent third party institute (Danish Technological Institute)
of 5 flows at 3 different temperatures (15, 50 and 85°C).
Based on these results, an optimized VFS 2−40 l/min has been developed and tested in the Grundfos lab. These results are also shown on the chart.
The
results presented as ”standard” are obtained at a third party institute. The
results presented as “optimized” are obtained in the Grundfos laboratory.
It can be
seen that the standard VFS 2−40 l/min sensor has an error in
percentage of reading below 4% down to typically 8 l/min. To improve this
according to the standard, we have optimized the VFS 2−40 l/min
performance in order to perform better.
The
temperature signal from the sensor has a maximum error of ±1 K
in the range from 25−80°C. Looking at this, it will most likely not be
enough given that a maximum error of a pair of ±2 K with a temperature
difference of 5 K will account for an error of 40% of measured value.
Grundfos
Direct Sensors™ have, with success, recommended that customers carry out an
alignment of the temperature measurement of the 2 sensors. The recommendation
is based on the fact that if the sensor pair offset is adjusted around a single
point the error margin in terms of the delta measurement around this point will
be considerably reduced.
In
practice, this has been carried out by allowing the circulation pump to run
while the heat pump is not running (or is in by-pass). Eventually, temperatures
at both inlet and outlet of the heat pump will be almost the same.
Using this
procedure a maximum delta T value of below 1°K is achievable
Looking into other solutions for temperature measurements the following demands should be met acc. to EN 1434:
Calculating this in percentage of measured value a maximum error of 3.5% occurs. To meet this demand completely, a pair of Temperature sensors of accuracy <0.2°K must be utilized. There are many suitable devices such as resistance temperature detectors (RTD’s), thermocouples and negative temperature coefficient thermistors (NTC’s) that can meet this accuracy requirement.
The data
from above have been used to calculate the total error for 2 examples to give a
more clear idea of the sensors performance in terms of the total error. In the
example, the system has a flow of 30 l/min and an inlet temperature to the
heating system of 65°C and 75°C respectively. The error contribution from the
temperature sensor pair and flow sensor is accounted for and the total error
has been calculated using the arithmetic sum of the errors.
Example 1:
(30 l/min, inlet temperature 65°C, return temperature 55°C)
Et
+ Ef = (1K/10K) * 100% + 1% = 11%
Example 2:
(30 l/min, inlet temperature 75°C, return temperature 55°C)
Et + Ef
= (1K/20K) * 100% + 0% = 5%
It is important to understand that these solutions have been on the market for more than two years with substantial energy savings reaped by some of the world’s largest heat pump manufacturers of air-water and ground source heat pumps. We call it Future Now, as we will continue to update our products to obtain even better accuracy of our products for present and future partner’s to enable an even more efficient control of and heat metering integrated into their heat pump.
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