6. Demonstration House
6.3 Measurements of the New Solar Combisystem
6.3.2 Energy Balance of the New Solar Combisystem
In the demonstration house the new solar heating system based on a condensing natural gas boiler was measured for a period of 4 months from October 2006 till January 2007. The monthly energy balance for this system is presented in Fig. 6–22 and Table 6–2. It is planned to collect the data further on till end of 2007 in order to get a full year. The definitions of the energy values and how they are measured are described in chapter 6.3.1 (page 115) and Fig. 6–20. The efficiencies are defined as:
1. Boiler Efficiency:
n Consumptio Gas
Natural
Boiler
η_boil = Eq. 6–6
2. Solar Fraction:
Boiler Solar
Solar SF
+
= Eq. 6–7
3. Natural Gas - COP:
n Consumptio Gas
Natural
Heating Space
n Consumptio Water
Hot Domestic
COP +
= Eq. 6–8
4. Hydraulic Efficiency:
Solar Boiler
Heating Space
n Consumptio Water
Hot Domestic d
η_hy
+
= + Eq. 6–9
“Boiler Efficiency” is the average efficiency of the boiler over a period and therefore including start/stop losses and standby losses.
“Solar Fraction” shows the share of energy delivered from the solar collector circuit compared to the total hydraulic energy supplied to the system. (Not the fuel consumption!) Therefore all heat losses have to be covered by the boiler and the solar collector in the same ratio as the solar fraction is calculated. As discussed in detail by (Heimrath 2004) with good arguments at least eight different types of solar fractions can be defined. The main problem is how to share the heat losses between the auxiliary heat source and the solar collector. Anyway, in order to evaluate a solar heating system the solar fraction itself is not sufficient. It is necessary to evaluate the whole system as well, which for example can be done with the following two key figures.
“Natural Gas - COP” is the coefficient of performance based on the natural gas consumption that is calculated with the low heating value. In the case of solar heating systems, the COP can be much greater than one, because of the gained solar energy.
In summer time, typically the COP is infinite thanks to 100 % solar fraction (if parasitic electricity for pumps, etc. is not taken into account).
“Hydraulic Efficiency” shows how much of the energy delivered from boiler and solar collector was really consumed in the house. The remaining energy is heat loss.
The result of the few months is presented in Fig. 6–22 and Table 6–2. In the period from October until middle of December, the controller program was evaluated, tested and improved in several steps. In addition, the technical unit was improved as well.
On the other hand, the climate and the user behavior are changing the boundary conditions daily. Therefore, several effects influence the key numbers in a positive or negative way and it is not possible to conclude exactly based on monthly values how much the changes in the system improved the system. In chapter 5 (page 69), based on shorter measurement periods these effects are discussed more detailed.
REBUS project Demonstration House - HELSINGE Energy Balance - 10-2006 till 01-2007 - Monthly Values
0 400 800 1200 1600 2000 2400 2800 3200 3600 4000 4400 4800
Oct 06 Nov 06 Dec 06 Jan 07
Energy [kWh]
0%
10%
20%
30%
40%
50%
60%
70%
80%
90%
100%
110%
120%
Efficiency [%]
Solar Gain [kWh] Natural Gas Consumption [kWh] Space Heating [kWh]
Domestic Hot Water-Consumption [kWh] Domestic Hot Water-Circulation [kWh] Electricity [kWh]
Boiler Efficiency [%] Natural Gas - COP [%] Hydraulic Efficiency [%]
Fig. 6–22 Energy balance for the new solar heating system in the demonstration house.
Table 6–2 Energy data for the new solar heating system in the demonstration house
Ambient Temperature Average Source: DMI Natural Gas Consumption Low Heating Value = 10.67 kWh/m3 Boiler Boiler Temperature Difference Monthly average Solar Gain Space Heating Space Heating Temperature Difference Monthly average Domestic Hot Water-Consumption Domestic Hot Water-Consumption Average daily consumption Domestic Hot Water Consumption Temperature Difference Monthly average Domestic Hot Water-Circulation Electricity consumption per day Solar Heating System Boiler Efficiency (Boiler/Gas) Natural Gas - COP (DHW+SH)/(Gas) Solar Fraction (Solar)/(Solar+Boiler) Hydraulic Efficiency (DHW+SH)/(Boiler+Solar) Circulation as Loss
Ta Gas Boiler Solar SH DHW Circ Electr. eta_boil COP SF eta_hyd
[°C] [kWh] [kWh] [K] [kWh] [kWh] [K] [kWh] [kWh/d] [K] [kWh] [kWh/d] [%] [%] [%] [%]
10-2006 11.7 1266 1222 16.8 143 1040 11.9 216 7.0 35.3 18 2.9 96.5% 99.2% 10.5% 92.0%
11-2006 7.4 2146 2127 15.6 66 1830 12.7 238 7.9 37.4 14 3.1 99.1% 96.4% 3.0% 94.3%
12-2006 6.6 2244 2233 16.1 27 1853 12.5 274 8.8 39.2 10 3.2 99.5% 94.8% 1.2% 94.1%
01-2007 4.3 2828 2766 15.9 30 2421 12.3 214 6.9 39.6 14 3.2 97.8% 93.2% 1.1% 94.2%
In October the hydraulic efficiency (last column in Table 6–2) was 92 % while from November until January the hydraulic efficiency was quite constant at about 94.2 %.
A reason for that most likely was one change which was done end of October: The cabinet of the technical unit was closed with the goal to reduce the heat losses of all non-insulated (except the heat exchangers) components inside the cabinet, which obviously was achieved.
Also the boiler efficiency in October was only 96.5 % where it was 99.1 and 99.5 % in November and December. A remarkable difference of the monthly natural gas consumption (1,266 kWh compared to around 2,200 kWh) is one typical reason for such a difference. The boiler efficiency typically is higher with higher load because of relative less heat losses due to less standby periods. On the other hand this effect is not very strong if the heat load is above 1,000 kWh, what can be observed clearly in several measurement reports (Furbo et.al. 2004; Wolff et.al. 2004). But two more reasons might influence this improvement. Due to the closed cabinet since end of
October of course also the standby losses of the boiler are reduced since the boiler is inside the cabinet. Additionally also since end of October the combustion air for the boiler is not sucked directly from outside but from inside the cabinet via the technical room and a hole in the wall. Due to this change the inlet temperature always is about 30 to 35°C instead of the ambient temperature. In fact this is a kind of heat recovery of the heat losses in the technical room and in the cabinet.
In January the boiler efficiency decreased a little to 97.8 % which is assumed to be due to the lower ambient temperature which forces the boiler to operate more often in the hot water mode (see 3.1.5, page 35) at the higher temperature level with a set point forward temperature of about 62°C instead of 52°C. Since the space heating temperature difference also in January was quite similar to the period before (around 12 K), necessarily also the space heating return temperature (following the forward temperature) was higher. This of course reduces the condensation rate quite strong (see 5.4, page 92) and therefore also the boiler efficiency.
In general it can be observed that the temperature difference of the boiler in all four months was around 16 K. Since the boiler always can charge the auxiliary volume in the solar tank, the boiler flow rate also is always quite constant: about 580 ltr/h.
Therefore it can be calculated that the boiler also is operating always at a quite constant power of about 11 kW in average. The monthly average space heating power during these four months was between 1.4 and 3.3 kW. This shows that normally the boiler would be forced most of the time to operate far below the minimum power of 5.7 kW, which can be reached by modulation. Therefore a quite high frequency of start and stops of the burner would occur and decrease the boiler efficiency and increase the emissions.
The contribution of the solar collector during these four months naturally was not very high, especially since the solar heating system (6.75 m2 collector area) is relative small for a solar combisystem. In October the solar fraction in total was 10.5 %.
Looking at the system as an oversized solar domestic hot water system and considering also the hydraulic efficiency of the whole system, the solar collector supplied about 61 % of the hot water consumption.
The following 4 months February till May (which are not measured yet) typically are those with much higher irradiation and therefore also the solar fraction should be higher.
The overall efficiency is the coefficient of performance (COP), assuming that the complete solar combisystem is a heating system, which is consuming natural gas to supply the demand of domestic hot water and space heating. In fact this is the major interest: to use as less as possible auxiliary fuel to satisfy the demand. This key figure will be most interesting after a full period of one year measurements because of the very uneven contribution of solar energy within one year.
Anyway, a coefficient of performance of 93.2 % and 94.8 % (in January and December with almost no solar contribution) compared to a conventional heating system is a quite good result.
A Danish study (Furbo 2004) presented monthly measurement results of a heating system in a one-family house with a condensing natural gas boiler. The coefficient of performance for example was 94.1 % at 2298 kWh natural gas consumption in April or 95.3 % at 2706 kWh natural gas consumption in October. The monthly boiler efficiency in this two cases was 96.6 % in April and 96.3 % in October respectively.
6.3.3 Specific Detailed Evaluation of the New Solar Combisystem