G. S. Aglietti, S. Redi, A. R. Tatnall, T. Markvart and S. J.I. Walker
University of Southampton United Kingdom
One of the major issues in the use of ground based photovoltaic (PV) panels for the large scale collection of solar energy is the relatively low energy density. As a result a large area is required on the ground to achieve a significant production. This issue is compounded by the fact that the power output of the devices is strongly dependent on the latitude and weather conditions. At high latitudes the sun is relatively low on the horizon and a large part of the solar energy is absorbed by the atmosphere. Countries situated at high latitudes, with climates such as the UK, are therefore challenged in their exploitation of solar energy as the average number of Peak Solar Hours (PSH – numerically equal to the daily solar irradiation in kWh/m2) is relatively low. In Europe, typical annual average PSH values for horizontal surfaces range from about 2.5 h in northern England to 4.85 h in southern Spain (Markvart & Castaner, 2003). As, roughly speaking, the cost of the energy produced is inversely proportional to the average PSH, northern European countries are at a considerable economical disadvantage in the exploitation of solar energy with respect to other regions. On the other hand, areas with high ground solar irradiations (e. g. African deserts, see Kurokawa, 2004) are remote from most users and the losses over thousands of miles of cables and the political issues entailed in such a large project, severely reduce the economic advantages.
A different approach to address most of the shortcomings of ground based solar energy production was proposed by Glaser et al., 1974 and his idea has captured the imagination of scientists up to this day. The basic concept was to collect solar energy using a large satellite orbiting the Earth. This satellite would be capable of capturing the full strength of the solar radiation continuously and transmit it to the ground using microwave radiation. The receiving station would then convert the microwave radiation into electric energy for widespread use.
The original concept was revisited in the late 90’s (Mankins, 1997) in view of the considerable technological advances made since the 70’s and research work on this concept is still ongoing. However a mixture of technical issues (such as the losses in the energy conversions and transmission), safety concerns (regarding the microwave beam linking the satellite with the ground station) and cost have denied the practical implementation of this concept. The latter is a substantial hurdle as the development of Satellite Solar Power (SSP) cannot be carried out incrementally, in order to recover part of the initial cost during the development and use it to fund the following steps, but it requires substantial funding upfront (tens of billions of dollars according to Mankins, 1997) before there is any economical return.
As a compromise between Glaser’s (SSP) and ground based PV devices it is possible to collect the solar energy using a high altitude aerostatic platform (Aglietti et al., 2008a, b). This approach allows most of the weather related issues, except for very extreme weather conditions, to be overcome as the platform will be above the cloud layer. As the platform is also above the densest part of the troposphere, the direct beam component from the sun will travel through considerably less air mass than if it was on the ground (in particular for early morning and evening) and this will further improve the energy output. Therefore this method enables considerably more solar power to be collected when compared to an equivalent ground based system. In addition, the mooring line of the platform can be used to transmit the electric energy to the ground in relative safety and with low electrical losses. Although this approach would capture between 1/3 and 1/2 of the energy that could be harvested using a SSP, the cost of the infrastructure is orders of magnitude lower, and this approach allows an incremental development with a cost to first power that is a few orders of magnitudes smaller than that necessary for SSP.
Most researchers up till now have proposed harvesting energy at high altitude by exploiting the strong winds existing in the high atmosphere such as the jet streams (Roberts et al., 2007). This would be achieved using Flying Electrical Generators, that are essentially wind turbines collecting wind power at altitudes from few hundred meters (www. magenn. com) to over 10 km.
The extraction of this energy using the type of machines proposed by Roberts et al. 2007, although feasible and most probably economically viable, is relatively complex in mechanical terms. One of the issues is that in low wind the machine (that is heavier than air) needs to reverse its energy flow and take energy from the ground to produce enough lift to support itself and the tether. Alternative designs like the MAGENN (www. magenn. com) overcome this problem using a lighter-than-air approach so that the buoyancy keeps it in flight all the time. However the mechanical complications are still considerable.
The exploitation of solar energy at high altitude may therefore be simpler in engineering/mechanical terms, and provide a very predictable/reliable source. One of the crucial steps to demonstrate the viability of the concept is a reliable calculation of the solar energy available as a function of the altitude. After a brief introduction on aerostatic platforms, the energy available at different altitudes is investigated. The concept of the Aerostat for Solar Power Generation (ASPG) is then described together with the equations that link its main engineering parameters/variables, and a preliminary sizing of an ASPG, based on realistic values of the input engineering parameters is presented.