3. Coefficient of Performance
6.4.6 Energy Savings
To estimate the energy savings based on measurements in practice on the one hand side is based on realistic system behavior but on the other hand side influenced by a lot of boundary conditions which are changing over time. In this specific case the most important variable boundary conditions (beside replacing the old heating system with the new solar combisystem) are:
• The weather conditions and therefore the heating degree days (HDD).
• The behavior of the occupants in terms of hot water consumption.
• The behavior of the occupants in terms of comfort wishes based on room temperature.
• Different use of the house since the basement is occupied during the measurements with the new solar combisystem and not with the old heating system.
• Different handling of the thermostat valves of the radiators due to intensive training of the occupants between the two measurement periods.
To estimate the energy savings for this four months period the following approach is used: Fig. 6–37 shows the coefficient of performance (COP) as a function of the total heat load for space heating and domestic hot water for both the old heating system and the new solar combisystem.
80%
82%
84%
86%
88%
90%
92%
94%
96%
98%
100%
1000 1500 2000 2500 3000 3500 4000
Monthly Load for Space Heating + Domestic Hot Water [kWh]
COP [%]
Demonstration Solar Combisystem Linear Regression Old Heating System Demonstration Old Heating System
Fig. 6–37 Coefficient of performance of the new solar combisystem and the old heating system for both as measured and as calculated based on linear regression.
For this graph only those data of the old heating system are used, which have been measured during space heating period with a total load more than 1,000 kWh. For the old heating system based on these monthly measurement points a linear regression curve is fitted in a way, that using the calculated COPreg based on the equation of the regression, the calculated natural gas consumption in total of all months results again in the same total natural gas consumption as measured. The equation of the linear regression COPreg is:
reg =0.9068−0.00002×Load
COP Eq. 6–10
Based on this equation finally it is estimated how much natural gas consumption would have been used in the old heating system (Natural Gas Consumptionreg) based on the measured heat load of the new solar combisystem. The difference to the
measured natural gas consumption (Natural Gas Consumptionmeas) of the new solar combisystem gives the energy savings. The result of this calculation is shown in Table 6–5.
Table 6–5 Estimation of the energy savings based on linear regression of the coefficient of performance (COP) of the old heating system.
Month Heat
Load COPreg Natural Gas Consumptionreg
Natural Gas Consumptionmeas
Energy Savings
[kWh] [%] [kWh] [kWh] [kWh]
Oct 06 1,256 88.2 1,424 1,266 158
Nov 06 2,068 86.5 2,388 2,146 242
Dec 06 2,127 86.4 2,461 2,244 217
Jan 07 2,635 85.4 3,086 2,828 258
Total 8,086 86.4 9,359 8,484 875
It can be observed that the energy savings of the complete solar combisystem of 875 kWh was 3.3 times higher than the solar gain of 266 kWh. The average hydraulic efficiency in these four months was 93.9 %. Therefore, about 250 kWh (266 x 0.939) or about 29 % of the energy savings are due to the solar gain and 71 % are due to the higher efficiency of the condensing natural gas boiler itself but also due to the high quality of system integration.
For the total measurement period from July 7th, 2006 until January 31st, 2007of the new solar combisystem only the natural gas consumption could be measured due to the problems with the energy meter until end of September 2006. But as an overview the natural gas consumption of the old and the new heating system can be given as shown in Table 6–6:
Table 6–6 Natural gas consumption for the period from July 7th until January 31st (208 days) for the old (2005/06) and the new (2006/07) heating system.
Natural gas consumption
Old heating system (2005/06) 14,618 kWh New solar combisystem (2006/07) 8,802 kWh
Difference 5,816 kWh
As mentioned before (see chapter 6.4.4 on page 134), additionally to the energy savings in terms of heat the new solar combisystem also was able to save about 8 % of the average daily electricity consumption, which is about 0.3 kWh less per day, about 9 kWh less per month, respectively.
In average the new solar combisystem consumed 3.1 kWh per day to operate the complete heating system including the space heating distribution system and the hot water circulation pump. This corresponds to an average power of 129 W or about 95 kWh per month.
According to the IEA-SHC Task 26 standard the energy savings are calculated based on a reference energy consumption depending on the total heat load. The so-called
“fractional energy savings” Fsav,therm is defined as (Weiss et.al. 2003):
Eref ESCS Eref
therm sav
= −
F , Eq. 6–11
ref
Qloss QSH
QDHW
ref η
+
= +
E Eq. 6–12
Qloss =0.00016∗ 0.75∗VD ∗
(
TT −Ta)
∗8760 Eq. 6–13Where:
Eref Final energy consumption of the (conventional) reference system [kWh]
ESCS energy consumption of the solar combisystem [kWh]
QDHW Domestic hot water load [kWh]
QSH Space heating load [kWh]
Qloss Heat losses from the reference heating system for one year [kWh]
VD Average daily hot water consumption [Liter/day]
TT Set point temperature of the hot water tank [°C]:
52.5°C is used for this in Task 26
Ta Ambient temperature around the hot water tank [°C]:
15°C is used for this in Task 26
ηref Efficiency of the auxiliary heater in the reference system [%]:
85 % is used for this in Task 26
According to the measurements of the new solar combisystem the following energy loads have to be taken into account for this period of four months to calculate the final energy consumption of the reference system:
QSH 7,144 kWh (see Table 6–3, page 130) QDHW 942 kWh (see Table 6–3, page 130) VD 174 Liter/day (see Table 6–4, page 132) The heat loss of the reference heating system therefore is:
( )
kWhloss 0.00016 0.75 174 52.5 15 2952 202
Q = ∗ ∗ ∗ − ∗ = Eq. 6–14
The final energy consumption for the reference system therefore is:
kWh
ref 9751
85 . 0
202 7144
E 942+ + =
= Eq. 6–15
The energy savings therefore are calculated to:
kWh
SCS
ref E 9751 8484 1267
E − = − = Eq. 6–16
The “fractional energy savings” Fsav,therm for this period of four months therefore is calculated to:
13%
9751 8484 9751
F , − =
therm =
sav Eq. 6–17
The energy savings of the two different calculation methods are quite different:
875 kWh compared to 1,267 kWh. The measured coefficient of performance in the conventional heating system in the demonstration house was 86.4 % were the assumptions in Task 26 lead to 82.9 %. One reason for this is that in Task 26 the annual boiler efficiency (85 %) is assumed for a full year operation, including the summer period which typically is decreasing the average efficiency. Additionally the measured annual boiler efficiency in the demonstration house was 90.4 %, therefore also significant higher than 85 % as assumed in Task 26.