Solar-gas boilers in successful trials
by W van Zanten, Novem, the Netherlands

A demonstration project using solar-gas combination boilers has been undertaken successfully in the Netherlands. The boilers not only save energy, but also improve user comfort. Using the experience gained in the project, the manufacturer has started production of a high-efficiency version of the boiler.


The system

In a solar-gas combination boiler, space heating and hot water production are combined in one device. The main part is a 240 litre water tank. The solar collector provides heat to the exterior of the tank, which stores hot tap water. The vessel also accommodates a gas burner in the middle and a heat exchanger coil in the top for space heating (see diagram). In the demonstration version, the burner has a capacity of 16.2 kW. The manufacturer, Atag/Luigjes (ATAG), is one of the first to build such a combination boiler and other Dutch manufacturers/institutions, such as Daalderop and Gastec, have started using the concept. An ATAG solar­gas combination boiler with a collector of 2.83 m2yields 4 GJ/year, and the 5.40 m2 collector system yields 5.64 GJ/year.


A test involving 25 solar-gas combination boilers showed a boiler efficiency of 90.1% (based on the lower heating value, ie without condensation). On average, the solar collector contributed 10.6% to this value (with a collector size of 3.1 m2 ). A survey among the 25 users showed equal performance in terms of ease of use, control, technical performance, noise, space heating, etc. The survey also showed that the best feature of the combination boiler its provision of hot tap water, because normally the capacity of the burner limits the use of hot water taps while the dwelling is heating up. The combination system provides hot water to the taps and radiators simultaneously, and so hot filling of items such as dishwashers and washing machines is uninterrupted. After one year of operation, three quarters of the users would consider buying a system.

Households that have relatively high hot water consumption take best advantage of the solar energy and therefore get the best return on their investment. However, the solar­gas combination boiler is also suitable for households with low hot water consumption, if used only for space heating ­ installing the hot water part of the system is too expensive to be feasible.

The demonstration project involving the 25 installations led to a number of improvements and a new high-efficiency system is now available commercially. Ten of these systems are undergoing a one-year field test, with the results expected at the end of 1999.


The ATAG solar-gas combination system consumes at least 180 m3/year less natural gas than an equivalent system. The project participants saved 650 m3/year of natural gas compared to their former systems. A solar-gas combination system costs NLG 6,800 (where NLG is the Dutch guilder), which is some NLG 1,200 more than the cost of a comparable system. The payback period is 13 years, assuming a natural gas price of NLG 0.50/m3.

For more information contact the CADDET Dutch National Team in Sittard.

The CADDET Renewable Energy Newsletter is a quarterly magazine published by the CADDET Centre for Renewable Energy at ETSU, UK.

The articles published in the Newsletter reflect the opinions of the authors. They do not necessarily reflect the official view of CADDET.

Enquiries concerning the Newsletter should be addressed to Pauline Toole, Editor, CADDET Centre for Renewable Energy, ETSU, Harwell, Oxfordshire OX11 0RA, United Kingdom. Tel: +44 1235 432968, Fax: +44 1235 433595.