Public lighting with solar energy
by W van Zanten, Novem, the Netherlands

Sometimes lighting is needed in areas where grid connection is too expensive. As a result, these areas remain unlit, even though the need is great. Recent innovations and cost reductions have now made PV lighting a viable option and experience in the Netherlands has proved this to be the case.

Public lighting along a dyke

Background

There are numerous locations where public lighting would be an advantage, but it is not possible because an electricity grid does not exist. In these situations, PV-powered lighting can now be a cost-effective solution. It can even entail lower investment and running costs compared to conventional lighting and the necessary grid extension. Suitable locations for PV lighting include remote car parks, isolated crossroads and T-junctions, and cycle paths. The reliability of such lighting is high, and the Netherlands has used this kind of public lighting for some years. The first projects featured single lampposts; at present there are single or multiple lampposts at more than 250 sites.

Project example

An example of a Dutch project concerns an isolated car park adjoining a motorway that runs between Maastricht and the Dutch city of Heerlen. Here, 12 PV lighting masts have been installed, each carrying six 115 Ah rechargeable batteries, charged by three 60 Wp PV modules mounted on top of the mast and facing south. To make the best use of the charge in the batteries, each mast has a 7 W and an 11 W fluorescent lamp in a specially designed fitting. When the batteries are fully charged, both lights are switched on after dark. When charged between 60% and 80%, only the 11 W lamp is switched on at the approach of a vehicle; when charged between 30% and 60%, only the 7 W lamp is lit. Below 30% charge, neither lamp is activated. This occurs only after 40  consecutive “dark” days, when it has not been possible to recharge the batteries. Following installation of the masts, the crime rate fell and this success has led to other car parks being fitted with similar systems. There are also plans to install PV lighting in public toilets.

Energy-efficiency in these systems is important to reduce investment costs, since lower energy consumption requires less PV and battery capacity. To achieve this, equipment such as energy efficient lighting, motion detection, daylight and time switches is used.

The system

PV panels generate electricity for consumption on-site and, as generation and consumption are not simultaneous, a storage system is needed. A typical system consists of PV panels, a battery, a light box, a light bulb, an automatic charging unit and automatic lighting control. The life-cycle of the PV panels is similar to that of conventional lighting equipment, ie 20–30 years. The system must be designed to have sufficient “autonomy” for the conditions in which it will work; autonomy being the length of time the system can operate between battery charges. In the Netherlands, for systems operating all year round, 40 days autonomy is usually considered adequate. The lifetime of the battery depends mainly on the charge control unit. This unit protects the battery against overcharging and excessive, heavy discharging. The lifetime of a battery depends on the supplier, but is usually up to 10 years.

To bridge the darker periods in a Dutch winter, a capacity of about 30 Wh/Wp is needed. As a rule of thumb, 110 Wp will provide enough power for an annual energy supply of 27 kWh. If the system is only used during summer, a capacity of 10 Wh/Wp will suffice. The solar panel capacity and the minimum battery capacity are calculated based on the requirements for a reliable lighting supply in January. This month usually has the lowest ratio of solar radiation to electricity need, ie the longest period between battery charging. The PV panel should be inclined close to vertical in January due to the low sun in Northern Europe. For applications in summer, a more shallow inclination is desirable. Efficient luminaires reduce energy consumption. Two lamps in one luminaire can provide light more energy efficiently than an electronic dimming system.

Public lighting along a cycle path

Economics

The investment cost of a PV lighting unit is NLG 5,000–12,000 (where NLG is the Dutch guilder). The costs of the mast, luminaires and installation are comparable with conventional lamp posts. The extra costs are for the solar panel, the control unit, a supporting frame and the batteries. Savings are made on energy costs and not having to extend an existing grid, including transforming the voltage. It is difficult to give general figures for these as they depend on the location, the condition of the soil and the transformer. In the Netherlands, cable costs are about NLG 50/m and the cost of a transformer is NLG 10,000–40,000. Avoiding the transformer costs alone makes PV lighting an economic proposition. In spite of relatively high maintenance costs, such as new batteries every 10 years, a PV lighting installation is economical if the grid extension required is more than 100–200 m.

Conclusion

PV lighting systems can provide a cost-effective solution for public lighting needs, even in densely populated areas such as the Netherlands. Remote sites where lighting is required for safety and security are ideal. The deciding factor is the distance from the electricity grid.

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.

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