The potential application of variable renewable energy supplies to increase the horticultural productivity of the Isle of Lewis, Scotland.

  • Mark Hewitt Bradley

    Student thesis: Doctoral ThesisDoctor of Philosophy (awarded by OU/Aberdeen)


    Key factors in using variable renewable energy to sustain crop growth were investigated using the Isle of Lewis as a case study. Methods investigated sought to exploit plants’ abilities to accommodate a variable solar input by supplementing it with variable renewable energy. The extant solar resource on Lewis was characterised. The mean ratio of photosynthetically active radiation (PAR) to solar radiation (SR) (fE) recorded in 2010 was 0.458. fE was found to be significantly different between the first and last hour of daylight and 12:00 GMT (F, (2, 33) = 7.98, p<0.001) and between winter and summer months (F, (1, 10) = 20.86, p<0.001). This supports the suggestion that fE decreases as the atmospheric path length decreases. Significantly higher mean fE was also identified for the cloudiest days (F, (1, 22) = 6.22, p<0.05). Supplementing sunlight with intermittent, artificial illumination powered using wind energy significantly increased the growth of Brassica hirta. 53.26% of the additional dry weight produced using fixed diurnal illumination was achieved with 35% of the energy using this technique. The dry weight of B. hirta grown with illumination timed with tidal streams was not significantly different from that grown using fixed diurnal patterns. This is potentially important for the use of renewable energy for horticultural illumination. The possibility of using energy to prioritise lighting in well insulated growing structures and the compatibility of electricity production and horticultural demand on Lewis were considered. These findings support the direct use of variable renewable energy to sustain crop growth and promote the concept of using plants to store renewable energy. This is of potential benefit for problems of renewable energy intermittency, the predicted need to increase world food supply and providing economic opportunities for remote areas with a poor solar resource but good supplies of variable renewable energy.
    Date of Award25 Mar 2014
    Original languageEnglish
    Awarding Institution
    • University of Edinburgh
    SponsorsUHI Greenspace
    SupervisorEddy (Eddie) Graham (Supervisor), Neil Finlayson (Supervisor) & David Harper (Supervisor)

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