ENERGY SAVINGS FOR A SOLAR COMBI SYSTEM

S. Furbo1* and A. Thdr2

1 Department of Civil Engineering, Technical University of Denmark, Brovej, Building 118, DK-2800 Kgs.

Lyngby, Denmark

2 AEE INTEC, Feldgasse 19, A-8200 Gleisdorf, Austria
* Corresponding Author, sf@byg. dtu. dk

Abstract

In July 2006 a new developed solar heating/natural gas heating system was installed in an old one family house in Denmark. The new system is based on 6.75 m2 solar collectors and a condensing natural gas boiler. Before the installation of the solar heating system, the house was heated by a non condensing natural gas boiler.

The heat demand, the electricity consumption for the energy and heating system as well as the natural gas consumption were measured before and after installation of the solar heating system. Based on the measurements the yearly energy savings of the solar heating system are estimated to 3600 kWh for 2005 and 4000 kWh for 2007, corresponding to 530-590 kWh/m2 collector per year. The energy savings will vary from year to year. In years with a high heat demand and a high solar radiation, especially in the spring, the energy savings will be high. In years with a low heat demand and a low solar radiation, especially in the spring, the energy savings will be low. The yearly heat production of the solar collectors is in 2007 about 2/3 of the yearly energy savings.

Keywords: Solar combi systems, energy savings, measurements

1. Introduction

Only few investigations on energy savings for solar heating systems in practice have been carried out, [1]. This is remarkable since most solar heating systems are installed with the aim to save energy. The reason for the few investigations is that it is extremely difficult to measure/document the energy savings for solar heating systems in practice.

In order to determine the energy savings, seven energy quantities/efficiencies must be considered.

Before installation of the solar heating system:

• Utilization of energy for the energy system.

• Electricity consumption for the energy system.

After installation of the solar heating system:

• Net utilized solar energy of the solar heating system.

• Saved energy by turning off the auxiliary energy supply system during the summer.

• Utilization of energy for the auxiliary energy supply system.

• Electricity consumption for the auxiliary energy supply system.

• Electricity consumption for the solar heating system.

Further, to make the determination even more difficult, the above mentioned energy quantities and efficiencies and thereby also the energy savings are influenced by the heat demand and hot water

consumption, which will vary from year to year due to weather variations, variations of user habits and due to changes of the hydraulics of the energy system.

A Swedish investigation of solar heating systems, based on questionnaires filled in by home owners, showed unexpectedly high energy savings for solar heating systems in practice [2]. Also unexpectedly large variations of the energy savings were reported. Solar heating systems with collector areas between 4 m2 and 25 m2 were included in the investigations. The reported energy savings ranged from 0 kWh per m2 to 2750 kWh per m2. The average collector area of the investigated systems was 11 m2, and typical energy savings ranged from 650 kWh per m2 collector to 900 kWh per m2.

A theoretical investigation showed that the energy savings of solar heating systems are strongly influenced by the efficiency of the energy system prior to installation of the solar heating systems

[3] . Especially the efficiency during the summer period is of great importance.

An investigation of new natural gas boilers and oil fired burners installed in one family houses without solar heating systems showed unexpectedly low utilizations of natural gas and oil in the summer and in periods with low heat demands [4]. This is of great interest in countries, such as Denmark, where oil and natural gas are often used as primary energy sources in houses, where solar heating systems are installed. A condensing and a non condensing natural gas boiler as well as a non condensing oil fired burner were included in the investigations. In spite of high yearly utilizations of natural gas and oil between 80% and 95%, the utilization of natural gas/oil decreased to values between 50% and 80% for the boilers/burner in the summer months. In the 5 summer months May-September the energy loss defined as the oil/natural gas consumption minus the space heating demand minus the hot water consumption was about 1000 kWh for the oil fired burner and the non condensing natural gas boiler and about 500 kWh for the condensing natural gas boiler. These energy quantities can easily be saved by well performing solar heating systems.

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