The following cases were defined and simulated to elaborate possible saving potentials when adding solar heating systems:

• The simulations were only carried out for Solar Domestic Hot Water Systems (SDHW).

• All simulations and comparisons were made for two boiler types. A condensing natural gas burner and an oil boiler, both with the same settings for calculating the boiler efficiency as described for Fig 8.

The reference system was assumed to be designed similar to the systems of the two monitored boilers described in the beginning [2]. This leads to the following key figures: Nominal power: 20 kW

Hot water tank: 160 litres, completely kept on set temperature

Insulation of the tank: 40 mm, thermal conductivity: 0.04 W/mK

Set temperature: 50°C with a hysteresis of 5K

Hot water tap temperature: 45°C

For the Solar domestic hot water systems, the following system design was simulated: Collector area: 2 m2 or 4 m2

Collector efficiency: Лс = 0.8; a1 = 3.12 W/ m2K; a2 = 0.012 W/ m2K2

Incident angle modifier: ke = 1 — xana(&/2); a = 3.3; for 0 = 50°: k% = 0.92

Hot water tank: 200 litres, auxiliary volume: 61 litres

Insulation of the tank: 40 mm, thermal conductivity: 0.04 W/mK

Hot water tap temperature: 45°C

The space heating system is always supplied directly by the boiler.

In Table 1 the monthly main data for the reference system with low hot water demand and a natural gas burner is shown. The space heating demand was defined to be 90 kWh/m2a, which in Denmark is a typical existing house. The average size of the one family houses is around 180 m2 which results in a yearly space heating demand of 16,200 kWh/a. Also it is assumed that there is no space heating demand during summer time, which in the Danish climate is the case from May till September.

Two domestic hot water demands (related to Fig 6 and Fig 7) were simulated, a high demand with 3,000 kWh/a and a low demand with 1,500 kWh/a. The monthly distribution of the consumption was simulated according to Fig 7. Additional variations were simulated with varying daily values between 80% and 120% of the average daily consumption during the week. The daily profile was created with different hourly domestic hot water consumptions as a percentage of the daily consumption, beginning at 5 a. m. and ending at 10 p. m. and with values between 0.8 % and 13.1 % resulting in 100 % for the whole day.

SHAPE * MERGEFORMAT

switched off because of no space heating demand during this period. Instead of the natural gas burner an electric heating element with an efficiency of 100 % supplies the missing energy to prepare the hot water.

In the case of “SDHW_1500_2m2”the total yearly energy saving per m2 collector area increased about 10 % compared to the results before presented in Fig 11.

The second group of simulations was done with an oil boiler as an auxiliary source. In Fig 13 the efficiency of the oil boiler used in different energy systems is shown. Compared to the natural gas burner the boiler efficiency in general is lower. In the summer period the difference is quite large. The lowest efficiency in August is less than 45 % in the reference case with low hot water demand (Ref_1500). Therefore the fuel reduction potential in this case is much higher. In Fig 14, the fuel reduction of the SDHW-systems

compared to the reference systems is shown. In Fig 15 the fuel reduction is shown for the case that in the summer period from May till September the oil boiler is switched off and an electric heating element with 100% efficiency is supplying the additional energy. In the case of “SDHW_1500_2m2” the total yearly energy saving per m2 collector area increased about 60 % compared to the results before presented in Fig 14.

Discussion

In Fig 16 a summary on yearly values of all calculated systems is shown. In the case of the small hot water demand of 1,500 kWh per year and a SDHW-system optimized for high solar gains (2 m2 collector area) the calculations result in the highest specific fuel reduction per m2 collector area: 960 kWh/(m2a).

This is twice as much as the pure solar gain of about 480 kWh/(m2a). The solar fraction of this SDHW-system is about 39 % for the whole year. In the summer period in this case the solar fraction is between 60 and 80 %. The solar fraction here is calculated by hot water demand minus boiler supply (= net utilised solar gain) and then divided by the hot water demand. This means that all the heat losses are covered by the SDHW-system.

The SDHW-system with 4 m2 collector area and 1,500 kWh hot water load leads to a solar fraction of 57 % for the whole year and between 90 and 98 % in the summer period. When increasing the hot water load to 3,000 kWh per year, the solar fraction is 47 % for the whole year and between 72 and 92 % in the summer months.

The results show very clearly that the lower the system efficiency of the reference system is, the higher the additional reduction potential of the fuel consumption by improving the system efficiency is when a SDHW-system is added to the system.

1100

Yearly energy saving potentials with SDHW-plants

Gas boiler Gas boiler + Electricity Oil boiler Oil boiler + Electricity

bOO

a

100 05 0 c Ш

1500 / 4mz / Fuel red. 1500 / 4mz / Solar Gain 3000 / 4mz / Fuel red. 3000 / 4m3 / Solar Gain 1500 / 2mz / Fuel red. 1500 / 2mz / Solar Gain 1500 / 4mz / Fuel red. 1500 / 4mz / Solar Gain 3000 / 4mz / Fuel red. 3000 / 4m3 / Solar Gain 1500 / 2mz / Fuel red. 1500 / 2mz / Solar Gain 1500 / 4mz / Fuel red. 1500 / 4mz / Solar Gain 3000 / 4mz / Fuel red. 3000 / 4m3 / Solar Gain 1500 / 2mz / Fuel red. 1500 / 2mz / Solar Gain 1500 / 4mz / Fuel red. 1500 / 4mz / Solar Gain 3000 / 4mz / Fuel red. 3000 / 4m3 / Solar Gain 1500 / 2mz / Fuel red. 1500 / 2mz / Solar Gain

Fig 16 Overview of all simulation results on a yearly basis

A short estimation on the basis of primary energy consumption showed that because of the high solar fraction in the summer period only a very small amount of electricity is necessary. Even in the case of the SDHW-system with the smallest solar fraction of 39 % per year the reduction of primary energy consumption still is approx. 880 kWh/m2 (assuming a primary factor 2.5 for electricity and 1.15 for oil [3]). Compared to 960 kWh/m2 mentioned before this is a very little influence. If the solar fraction of the SDHW-system increases the difference between the reduction potential of final to primary energy decreases to increasingly smaller values.

In principle a solar thermal system has a very little demand for maintenance. Mainly there are 3 parts where some work is necessary. The first point is to check the glycol and to change the liquid maybe all 5 to 10 years. The second point is the pump which can
perhaps break and then has to be replaced. The third point is the collector sensor and the controller which can be destroyed by lightning in the case of bad luck. On the other hand if the boiler can be switched off 40% of the year, the lifetime of the boiler will probably increase. In general the advantages and disadvantages will be more or less of the same magnitude.

The operating time of the solar pump is typically in the range of about 1.500 to 2.000 hours per year. If we calculate a pump with 50 W power, this leads to an electricity consumption of about 75 to 100 kWh per year. This is about 4%-10% of the fuel reduction. The summer months May till September have 3.672 hours. If in the reference case the boiler consumes 20 W in average (for standby and running two to three times a day for really supplying energy; also see Fig 3) this also leads to 73 kWh only in the summer period. This means, if there is additional electricity consumption, it is definitely much lower than 4%-10%.

Summary

In this paper the realistic behaviour and efficiency of heating systems were analysed, based on long term monitoring projects. Based on the measurements a boiler model was evaluated. Comparisons of measured and calculated fuel consumptions showed a good degree of similarity. With the boiler model, various simulations of solar heating systems were done for different hot water demands and collector sizes. The result shows that the potential of fuel reduction can be much higher than the solar gain of the solar thermal system. For some conditions the fuel reduction can be up to the double of the solar gain due to a strong increase of the system efficiency. As the monitored boilers were not older than 3 years, it can be assumed that the saving potential with older boilers could be even higher than calculated in this paper.

References

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[8] S. Knudson, Consumers influence on the thermal performance of small SDHW-Systems — Theoretical Investigations, BYG. DTU, Denmark; Solar Energy, Volume 73 , Issue 1, July 2002, pages 33-42; www. elsevier. com/locate/solener

[9] W. Streicher, et. al., Simulation programm SHWwin, TU-Graz, 1999, www. iwt. tugraz. at/de/main. html => Solarthermie => downloads => SHWwin