by Adrian Lea
With soaring energy prices, challenging emissions targets and the demand for greater security of supply, any imaginative and effective new way of harnessing renewable energy should be welcome. The installation of solar panels on former landfill sites could be exactly what's needed.
Previously such installations have proven difficult because of stability problems caused by differential settlement. However new technological advances are now making this combination perfectly possible - bringing attractive new revenue opportunities and after-use options.
Ground-mounted solar PV is becoming increasingly common throughout the UK, particularly in the south where higher solar irradiation levels often result in greater electricity yields. Mainly located on agricultural fields, solar farms can vary from 5 to 50 MW in size, covering areas between ten and a hundred hectares. Typical capital development costs range from £1 million to £1.3 million per installed MW for fully operational assets.Return on investment
The economics of developing solar PV projects are variable, with the largest and least complex projects normally being the best. Project returns of 8-9% are possible for third party long term investors, with revenues generally based on a Power Purchase Agreement for exported power, plus payments under the Renewables Obligation Certificate (ROCs) or Feed in Tariff (FiT) regime. Returns for landowners able to finance and operate their own projects will typically be higher, and considerably more if the power that's generated significantly replaces imported electricity from the grid.
Yet there has been understandable criticism about the use of agricultural land for solar farms, and the government has indicated its preference for 'brownfield' land being used instead. Waste management facilities, and former landfill sites in particular, naturally fall into this category. But the technical issues involved in developing ground-mounted solar generation on landfill sites - specifically to do with differential settlement - have proved a stumbling block in the past.
Differential settlement occurs at varying rates across an area as a result of the variations in the depth, type, method of waste disposal and decomposition rate within the underlying landfill. Its effects would in the past normally have resulted in damage to solar panel frames and foundations, ultimately leading to failure of the panels themselves as well as creating an unsightly layout.
In the past it would also have been hard to anchor the structures, since former landfill sites are often covered by an engineered cap in order to prevent the ingress of water and the escape of landfill gas.
Now, however, new technological advances in panel mounting and framing systems give landfill operators a way around these problems. Juwi, for example, a German solar PV supplier, has developed a bespoke design for frame-mounted solar arrays on landfill sites using a telescopic racking system that keeps panels flat and level. Its mounting mechanisms can be applied with minimal impact on the integrity of the underlying engineered cap and without applying undue loading to the landfill system itself. The solution has been deployed on a number of solar farms which have been developed on closed landfill sites in Germany.Commercial Arguments
There are many persuasive commercial arguments in favour of extending the role of former landfill sites to include the generation of solar power. The first and most obvious arises from the fact that many former landfill sites had a connection to the electricity grid in order to export electricity from landfill gas engines and, as landfill gas generation diminishes, grid capacity potentially becomes available.
The availability and capacity of an existing grid connection is normally a significant factor in the development of a large scale solar PV facility and could therefore considerably reduce both cost and timescale. The connection could potentially be optimised by using the solar farm to generate and export electricity during daylight hours while still utilising the landfill gas engines at night time.
Former landfill sites may well contain slope angles that favour the southerly orientation required for optimal performance of solar panels, as well as the large spaces ideal for the cost-effective development of a large scale solar facility. Previous landfilling operations probably also mean that issues associated with overshadowing by vegetation, buildings and overhead wires can normally be avoided.
Furthermore, such sites are often securely fenced, a pre-requisite for solar farms that contain thousands of valuable solar panels and where access needs to be controlled to what is essentially an electricity generating facility. Site monitoring and security may already be in place, while highway access for construction traffic and deliveries is also very good in most cases. Former landfill sites are also often located away from areas of environmental or cultural sensitivity, and are usually well screened from sensitive receptors such as residential properties.The Hickory Ridge Landfill in Atlanta, Georgia features a cap with an enhanced geomembrane anchoring system developed by HDR Planning
There are of course planning issues involved in developing a solar farm on a landfill site which require the normal close engagement with the planning authority
The key issue is usually landscape and visual impact. The raised profile of a former landfill site, while necessary to promote surface water run-off and minimise water ingress, may mean that a proposed solar farm could be visible over a wide area. The impact of this and possible mitigation such as site layout and landscape planting will require careful assessment, often utilising computer modelling.
The construction of the solar panel array must take into account the unique considerations of building on a former landfill. Maintaining the integrity of the cap and the gas management system are primary concerns.
The construction considerations include limiting the depth of excavation (depending on the depth of the restoration soils), avoiding the use of heavy equipment on certain areas, restrictions on laydown areas, maintaining erosion and surface water run-off controls, soil and/or groundwater testing if hazardous materials have been landfilled, robust safety measures and capping system repair plans.
There are a number of site development, ownership and operating models that can be tailored to suit individual circumstances. If 'client owned', the landfill operator/owner serves as the developer, financier, builder and owner of the PV system - enjoying all the direct benefits including electricity generation revenues but accepting all the project risks.
Under a 'land lease' model, the client selects a vendor to be responsible for the design, finance, building, ownership, operation and maintenance of the system. The vendor assumes all risks but claims much of the project revenue. Under a Power Purchase Agreement (PPA), the client buys the electricity generated by the PV array from the developer/owner at a predetermined rate. The client gains from the predictable price of power, while the project owner gains a fixed revenue stream to finance project development, installation and operation. This arrangement may be attractive where a landfill site has no grid connection (or one with insufficient capacity) but is located close to a material recovery facility or other similarly power-hungry facility.Other Renewables
Other renewable energy technologies are of course available! Wind turbines can be used in conjunction with solar, particularly where their generating potential is designed in harmony to optimise the use of the grid connection.
Care is obviously needed when considering the installation of a wind turbine on a former landfill site to avoid areas of previous landfilling where possible - or face challenging and expensive ground investigations and foundations. Wind turbines also normally face different planning issues including ecological, radar and landscape/visual impact.Conclusions
Extending the role of former landfill sites with the addition of large scale solar PV facilities can bring big potential benefits. Before you go too far, though, it will pay to get the right expert knowledge and technical support on your side.
Wasted effort and unnecessary costs and delays can be avoided by gaining a detailed understanding of the planning process, the business and financial issues, and the engineering aspects of combining solar generation with landfill management.
Adrian Lea is technical director at environmental engineering consultancy, Wardell Armstrong