Critics of utility scale solar developments often point to the amount of productive land taken up by solar arrays, but there are many opportunities for dual purpose land use, says GSES.
Recent months have seen the approval of many landmark utility scale solar developments, such as the 2 GW proposal in Bulli Creek, NSW, and the 130 MW proposal in Clare, QLD. These developments will take up vast amounts of land: the 130 MW solar farm will take up 340 ha of cane sugar fields, and the 2 GW site over 5,000 ha of primarily cattle grazing land.
As the number of these large scale greenfield developments increases, so too arises the argument that these developments are encroaching on existing land use. Solar developments occupy large tracts of valuable land that may otherwise be used for agricultural activities.
However, solar farms can be compared favourably to alternative uses such as mining. Not only can solar farm sites be easily rehabilitated at the end of their project life, but it is also possible for solar farms to offer dual purpose land activities, providing land owners with an opportunity to diversify their land use and increase the overall value and productivity.
While solar farms have large land footprints, not all of the land is actively taken up by solar panels or related infrastructure. Typically, modules in solar farms are installed on framing systems mounted on piles or concrete ballasts. Disturbance to the ground is usually less than 5 per cent of the area used, and only around 40 per cent of the surface is over-sailed by solar modules (BRE, 2014). As solar modules are tilted and raised on posts to avoid shading, the land beneath the module, as well as unshaded land between rows, is still available for plant growth, allowing for agricultural activities such as grazing and cropping.
Grazing of livestock within solar farms is the most popular dual use option, especially since this practice has the additional benefit of controlling vegetation growth. The presence of solar modules does not affect stock density, and provides shelter to grazing livestock.
However, consideration needs to be given to the choice of livestock. Sheep are common (see image above) as they are small enough to easily pass between rows of modules and do not damage equipment. For similar reasons, poultry such as chicken and geese are also common choices.
Grazing of larger livestock such as cattle and horses in solar farms has been attempted; however, the mounting system must be designed to accommodate their greater size.
Solar farms can also be combined with crop production. Planting of vegetables or non-food crops can occur underneath the solar arrays. This practice is also referred to as solar sharing or “˜agrivoltaics’.
In Japan, the practice known as “˜solar sharing’ is allowed on farmlands, where small, typically 100 W, solar modules are mounted three to five metres above ground and installed at spaced intervals with shading rate of up to 32 per cent to allow sufficient irradiation for the ground crop (see image below). The mounting structures are designed to allow space for tractors and other farming equipment, allowing farmers to receive a feed-in tariff from electricity generated while being able to continue farming without modification to their normal practices.
As Japanese regulations require the removal of these systems if they reduce the yield from farmlands by more than 20 per cent, the packing density of these solar farms is much lower than conventional solar farms (Movellan, 2013).
A study has been done in southern France with a set-up similar to “˜solar sharing’, but with varying packing density of modules. The result shows that the density of PV will affect not only the amount of irradiance available, but also the micro-climate underneath the array. Array orientation is also an important factor, as orientation will affect the shading pattern on the area below (Marrou et al, 2013).
However, there is still much research to be done on the exact effects of partial shading of crops from PV arrays, as well as the adaptability of different crops to these conditions.
Another way for integrating crops into solar farms is via strip-cropping of high-value vegetables or non-food crops under or in between rows of the solar array. An example is the planting of hardy crops such as agave or jojoba under the drip lines of solar arrays in arid locations. This takes advantage of water run-off from rain and cleaning solar modules.
Planting in this way requires careful layout of the solar array with regard to the proposed size of farming machinery required and the expected height of the crop.
Solar farms also present the opportunity to enhance the biodiversity of a site, especially if the site previously supported monocrops. Native wildflower and bird seed mixes can be sown between and around rows of modules, providing food and habitat for local birds, small mammals and invertebrates. Planting of species that have high pollen and nectar yields also presents beekeeping as an additional revenue option.
Constraints to dual purpose land use
During design and construction of the solar farm, consideration must be given to the intended agricultural activity and its requirements.
For example, soil compaction and damage to watercourses during the construction phase should be avoided to maintain the site’s capacity to support vegetation. The layout of rows of solar array and field margin will need to anticipate future maintenance requirements, taking into account the size, reach, and turning circle of machinery that might be used to maintain grazing pasture or tend to crops. Securing and protection of exposed cables, as well as the depth of buried cables, must also be taken into consideration where agricultural machinery and livestock will be present.
Dual use of land for agriculture and solar farming can add attractive revenue streams for land owners and plant operators. In the case of grazing livestock, dual use of land can also have a positive impact on the operation costs of the solar farm. Incorporating solar farms with livestock grazing, cropping, or biodiversity enhancement does, however, introduce more complexity into the design, operation and maintenance of a solar farm, and the costs of this must be considered.
In addition to the positive impacts outlined above, the benefit of increased use of available space for agriculture is felt not only by the land owner but also by communities, as the land creates a new identity and purpose for itself.