PV/CHP stand-alone energy system
by K Hofstad, Norwegian Water Resources and Energy Directorate

A pilot project in Norway aims to demonstrate that a new stand-alone energy system for remote dwellings is a feasible and cost-effective alternative to grid connection. The system meets all the energy needs, including electricity and space heating.

Background

Norwegian utilities have an obligation to supply electricity to all customers within their licensed area. If a connection has to be discontinued because it is too expensive to maintain, a feasible alternative supply must be established. Traditionally, diesel generator sets have been used in some remote areas. However, in conjunction with other partners, the Norwegian Water Resources and Energy Directorate has developed a new stand-alone power generation system to meet all the energy needs of an ordinary family house. The system is based on liquid propane gas (LPG) and PV modules as the primary energy sources.

System description

The system comprises:

  • a micro CHP unit (5.5 kWe/12.5 kWth) with an LPG-fuelled engine;
  • a PV generator (2 kWp);
  • a dc/ac inverter (220 V);
  • electricity storage (battery unit);
  • heat storage (water tank) (46 kWh).

A schematic of the system is shown in Figure 1. The electricity produced by the CHP unit and the PV generator is not used directly but stored in the battery unit, while the heat output from the CHP unit is stored in the water tank. The system can thus operate independently of user demand. LPG is also used directly, eg in a gas cooker. The hot water is mainly used for space heating and domestic hot water supply.

Operating principles

The CHP unit does not operate continuously, but starts and stops automatically depending on the charge in the battery, starting when it falls below 60% of full capacity. The CHP unit normally operates primarily to produce electricity. However, in winter, when the space heating demand is at its highest, the CHP unit is turned on to meet this requirement. When this happens, the excess electricity is used in an electric boiler in the hot water tank.

Influence of the PV generator

In summer, the electrical load is expected to be supplied entirely by the PV generator. In winter, the PV contribution is negligible and electricity is produced by the CHP unit alone. Electricity consumption is relatively constant throughout the year (Figure 2). In contrast, heat consumption is dependent on the time of year and is at its peak in the winter. If the CHP unit is running to produce electricity in the summer, when the heating demand is low, the heat it produces would be lost. However, by adding a PV generator to the system, the CHP unit can be switched off in sunny weather. This is shown in Figure 3, where the curve representing PV production is based on the average variable irradiation and cloud during a year. In this way, the PV generator and the CHP unit complement each other and improve the energy efficiency of the system.

Economics

The total price of the system is about NOK 440,000 (where NOK is the Norwegian krone). The running costs are mainly for LPG and replacement of the battery system (every 510 years).

The running costs can be influenced by the user. For a stand-alone system, electricity is far more expensive than oil and gas. Therefore, it is important to restrict electricity consumption to a minimum. A typical Norwegian household uses 20,000 kWh/year of electricity. Most of this electricity is used for space heating and hot water, and could be replaced by other energy sources; for example, an electric cooker can be replaced by a gas cooker that uses LPG. In this pilot project, electricity consumption is reduced to around 4,000 kWh/year and further reduction is still possible.

 

The annual cost, including the capital cost (at 7% interest), is estimated at NOK 70,000. Compared with the total cost of establishing and operating a grid connection, the cost of a stand-alone system for a single house is competitive if the distance to the existing grid is more than 2 km. A stand-alone system can also be an economic alternative in areas with a weak grid connection. In this case, the system can operate as a grid-connected local production unit in order to reduce the need for costly reinforcement of the connection.

Whether or not a PV hybrid stand-alone system is fully competitive with more conventional systems depends on several technical and economic factors. Currently PV would be commercially attractive in some cases but not in others. However, as the market for PV develops and the cost of components falls, PV hybrid systems are likely to become increasingly competitive in this type of application.

For more information contact the CADDET Norwegian National Team in Rud.

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.