HOT|COOL NO. 4/2017 "Technical Innovation and Optimization"



The increased thermal length (named TL1 and TL2) of the new heat exchangers are approximately a factor 1.4 compared to the old ones, equivalent to number of thermal units (NTU), increase from 3.5 to 5 (design conditions). An important issue to include in the analysis is the operational condition of the secondary side of the heat exchangers. As the experiment was made in a house operating under real conditions, there are to some extent variations in the parameters that are dependent on the supply networks as they are outside the system boundaries, mainly DH supply temperature and cold-

In the same way, as for the heating circuit, the temperatures are measured directly at the heat exchanger for DHW. The increase of thermal length is again approximately a factor 1.4 equivalent to NTU increase from 4.7 to 6.5 (design conditions). To get an energy meter based estimation of the impact of increased thermal length, the flow weighted data from the energy meter was used. An example of the results can be seen in figure 4.

water temperature. Furthermore, the flow rate of the DHW was varying according to the actual usage. Parameters within the system boundary, like pre- setting of the TRV’s (thermostatic radiator Valverde), set point and the setting of the DHW temperature were optimized prior to the experiment and kept fixed during the experiment. As the experiment was running for 7 months, it shows the general impact the new heat exchangers had on the operation.






In figure 3, the reduction of the primary DH return temperature related to the heating circuit temperature and the LMTD (logarithmic mean temperature difference) is shown as a function of the ambient temperature for the two heat exchangers. At a typical ambient temperature of 6°C, the primary return temperature was 29°C for the TL1 case and 26°C for the TL2 case. (Not shown in figure 3).

Figure 4: Left: Temperatures measured directly at DHW heat exchanger, to the right the daily energy consumption for DHW.

At August 11, the heat exchanger was changed, and the outcome was a reduced average primary DH return temperature of 4.0°C due to the longer thermal length. From the figure, it can be seen that the average primary DH supply temperature and the energy consumption for DHW production was similar, justifying the general conclusion of 4.0°C reduced primary DH return temperature. The missing data during July are due to summer vacation, and thus there was no use of DHW during that period. It is common for DH utilities in Denmark to apply a bonus scheme to encourage customers to achieve a reduced DH return temperature. In this specific case, a one percent saving is given on the variable cost of the energy for every degree the DH return temperature is reduced below 35°C. The end customer energy cost was 62.90 Euro/MWh, and with a weighted average reduced return temperature of 3.2°C, the yearly DH end customer cost saving is approx. 32 Euro. For the DHW TL2 case, the heat exchanger has additional 14 plates due to obtaining similar pressure drop, resulting in a cost increase of 90 Euro for the heat exchanger. For the heating circuit, the cost increase is 0 EURO, simply due to the same number of plates. This results in a direct payback time of 2.7 years, when considering the income due to reduced DH return temperature based on DHW and heating as well. The recommendation based on the conclusion of this case is to increase the thermal length of the heat exchangers both related to specifications of new systems and related to replacement at service. Looking at the remaining lifetime of the heat exchangers, where 15 years' lifetime is estimated, the net profit is 400 Euro.

Figure 3: Measuring data from heating circuit, primary return temperature and LMTD as a function of ambient temperature.

When estimating the representative reduction of the primary return temperature, the temperature must be weighted by the flow. Furthermore, to get a representative yearly estimate, the distribution of degree days must be taken into account over the heating season as well. The weighted reduction of return temperature is 3.0°C (October to April, 7 months) and this is the case for the reduced LMTD as well. Another takeaway from figure 3 is that a higher heat demand results in a higher reduction of the primary DH return temperature. Thus, at lower heating demand the impact of increased thermal length is negligible, as expected.


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