My hobby - Heat pump

1.     My hobby corresponds to my original profession – research in chemical engineering.  The heat pump Carrier 15 KW of heat, the supplier AC Heating Pilsen http://www.ac-heating.cz/  (Fig.1) was installed on December 1st, 2008 and replaced thus the 15 years old electric accumulation boiler having 2,4 m3 of water and using a night current.


2.     Double storey house with 130,7 m2 of heated ground floor, 585 m3 of heated volume and total 1458 m3 of building volume has 18 rooms with the floor heating. The house is cellarless and has unheated 2 atria, 2 unheated pantries, 2 garages and attic. The side wall consists of 25 cm air brick block, 5 cm of foam polysterene and (at the outside) the 15 cm of a solid brick. The calculated heat loss of the house is 14,8 KW at -12 oC. The actual value is approx. 11-12 KW. The set temperature is controlled by a temperature sensor in the hall of the ground floor. The temperature is measured also by a mercury thermometer with a precision of 0,1 oC.

3.     The pipes with the cooling agent R-410A from heat pump cross the garage wall and enter the „Hydrobox“ (600x600x210 mm), that contains plate heat exchanger Alfa Laval, circulating pump and a part of electronic circuits. The return water pipeline includes the Honeywell filtr 80 microns, reduction valve, manometer, safety valve, 12 liter expansion tank, the inlet for watering the system and ultrasound flowmeter and heat meter - „Ultrasonic Compact Heat Meter CF Echo II“ https://www.itron.com/PublishedContent/F31773-CF-ECHO-II_EN_10_09.pdf http://www.itron.cz/cs/teplo/

4.     The electricity setup contains a single phase electricity meter, the ampere-meter and the defrosting heating cable of the evaporator collector. 

5.     The data measurement is done once a day, usually in the morning before or after the electric current supply interruption (7,15 – 8,15 h). The measurement results are presented as the dependence on time or on the outdoor temperature. 

6.     At Fig.2 there are shawn the inlet (yellow) and outlet (blue) temperatures of water from heat pump in dependence on the outdoor temperature. It can be observed from the picture, that the outlet temperatures from the Alfa Laval heat exchanger are greatly decreased if the outdoor temperature is lower than minus 5oC. The output of the heat pump is already not sufficient at such low temperatures. Eventhough, we have not yet installed an additional electric heater to the heating circuit.

Fig. 2

7.     The amount of the heat supplied is calculated by a heat meter Echo 2 from the water flowrate and the difference of temperatures at the inlet and outlet of the heat exchanger. It can be observed from Fig.3 that the amount of heat supplied is decreased at the lowest outdoor temperatures. The amount of heat supplied is by cca 2 KW lower than the design values of AC Heating company (yellow curve).  A similar dependence shape holds also for the input electric power of a heat pump. At the lowest outdoor temperatures there is not even used the maximum possible power input of heat pump KWe=20A*230V=4,6 KW. A probable  reason for that is that the incoming air is not able to evaporize the necessary amount of liquid coolant. Both dependencies have a high scatter, namely at higher outdoor temperatures. The main reason of that is a spread in demands of the system for a heat supply. The house can be either overheated or underheated at the particular time. For example, after the morning electricity supply interruption (7,15-8,15 h) the house is underheated and there is a higher need for heat supply after the resumption of heating.

Fig. 3

8.     The local instantaneous values of the heat output and the electricity consumption presented at Fig. 3 have too high spread and do not describe the actual functioning of a heat pump in a sufficient way. Therefore, the average heat output delivered during 24 hours (calculated from the difference of GJ values measured by a heat meter) were introduced in Fig. 4. Calculated KW of heat have lower spread and approach the zero value at the highest temperatures.

Fig. 4

9.     The electric energy consumption KW hours per day is presented at Fig. 5 in dependence on the outdoor temperature. The maximum value of the electric energy consumption is  90 KWh per day.

Fig. 5

10.  At Fig. 6 there is presented the instant value of the heat factor TF, that is a proportion of heat output and electric input of individual measurements. It is apparent that TF=2 even at the lowest outdoor temperatures and it is meaningfull to use heat pump.

Fig. 6

11.  At Fig. 7 is presented the Coefficient of Performance COP, that is a proportion of heat delivered during 24 hours and amount of electricity consumed during the same period. Average COP values are higher than TF values, because the TF values relate to the morning measurements with temperatures lower than the average day values.

Fig. 7

12.  At Fig. 8 there are presented the instantaneous TF values and house and outdoor temperatures during the heating season 2009 – 2010. At first, the inner temperature was set to 22oC and since December the temperature was increased to 23oC. The reason was that is easier to keep the average temperature 22oC in the house. There were two periods with very low outdoor temperatures -15oC (20.12.2009 a 27.1.2010). It follows from the picture that the inner temperature dropped to 20oC. Currently, we do not have the additional electric heater in the water circuit and, if necessary, we have used 3 local electric heaters with total 6 KWe. It is a nuisance of the heat pump tariff D56d that the connection of heat pumps is blocked for 2 hours each day (8,3% loss of capacity). It is also apparent from Fig. 8 that the course of heat factor TF follows the course of outdoor temperature. The minimum value of TF is not lower than TF=2.   

Fig. 8

13.  The heating season 2009 – 2010 lasted 257 days, since Sept 27, 2009 to June 11, 2010. The heat pump consumed 8,949 MWh of electricity and produced 96,68 GJ of heat. The calculated value of the integral Coefficent of Performance COP=96,68/8,949/3,6=3,001. At Fig. 9 is presented the course of outdoor temperature and  COP dependences on time.

Fig. 9

14.  The final remarks. A heat pump purchase is a rather high investment that should be returned by savings of electricity consumption. Therefore, I have followed in detail the function and energy efficiency of the heat pump, namely during the first heating season 2008-2009. I have even observed the decrease of the heat factor caused by a gradual loss of a coolant. The coolant was completely lost after 39 days since a start-up, due to a bad metal quality of the valve casting. The second coolant loss happened 127 days since start-up, its cause was a poor seal of copper pipes to the heat exchanger. The third minor defect (653 days since start-up) was a very small water leak at the top water outlet of the Alfa Laval heat exchanger. The whole Alfa Laval heat exchanger was replaced by a new one. All three repairs were serviced promptly after the problem announcement.

15.  Heating costs 2010/2011