The Earth receives an incredible supply of solar energy. The sun, which has been burning for over 4 billion years, provides enough energy in one minute to supply the world’s energy needs for one year and in one day provides more energy than our current population would consume in 27 years.

In fact, the amount of solar radiation striking the earth over a three-day period is equivalent to all the energy stored in all fossil energy sources. This suggests that we should be putting considerably more effort into harnessing it and in Scotland researchers are doing just that.

Harnessing solar energy

There are two main technologies that harness solar energy. The first –perhaps the simplest and most established in Scotland –is “solar thermal”. Here, the sun’s heat or radiation is used to heat up a fluid – normally water mixed with anti-freeze. This is pumped through a heat exchanger that is connected to a domestic or commercial hot water supply. The fluid is contained within piping mounted on an “absorber plate” in a flat panel, which has a clear cover designed not just to let the maximum amount of sunlight through but to stop heat escaping back out again. The panels are mounted preferably on a south facing roof. In good conditions it can certainly be used to bring a domestic hot water supply up to full temperature, and at a minimum solar thermal is an easy way of at least pre-warming water so that it requires less conventional energy to bring it up to temperature.

Solar cells

The second way of using solar energy is to generate electricity with it using solar cells. A solar cell is a solid state device that converts sunlight directly into electricity by the photovoltaic (PV) effect, thus creating a voltage and a corresponding electric current in a material upon exposure to light.

Perhaps the best known material used for manufacturing solar cells is the silicon wafer, which is used to make the familiar dark blue PV panels. Silicon is still the subject of an intense research effort aimed at improving its efficiency, reducing its cost through improved manufacturing techniques and increasing the breadth of applications for solar cells.

In particular, considerable effort is now going into the development of “thin film” solar cells which are manufactured by depositing one or more thin layers of photovoltaic material (including silicon) onto a cheap substrate or backing sheet. These considerably lighter thin film cells allow flexible technologies to be developed, which can be moulded to the shape of a roof or even used for “wearable” solar cells fabricated with textile substrates. A form of semi-transparent thin film solar cell has also been developed for use as a type of glazing on buildings and has even been used on the wings of solar powered aircraft and on electric vehicles. Here again Scottish researchers are involved in the development of this technology.

Novel technologies such as dye-sensitised and organic (or “plastic”) solar cells also offer the possibility of cheap and efficient solar energy because of their low-cost materials and simpler manufacturing process compared to traditional solid-state cell designs. It can also be produced as a flexible sheet and is considered to be fairly robust. However, due to its less efficient nature it is currently being targeted at consumer electronics applications. The prize of developing even better dye-sensitised or plastic solar cells is extremely large and understandably there is therefore a major effort – in which Scottish researchers are involved – being put into the development of new dyes and organic semiconductors to allow construction of solar cells with better light-harvesting efficiency and stability.

A key goal of the PV industry is to realise technologies with suitable cost and performance to enable them to compete with fossil fuel electrical generation by achieving grid parity. Grid parity being “the point at which alternative means of generating electricity is at least as cheap as the retail price of grid power”.

Modifying the wavelengths of sunlight

Another important area of research and development in Scotland involves the concentration of solar energy on the photovoltaic surface. This means less photovoltaic material is required reducing both the size and cost of the solar cell overall. Here the strengths of the optics and photonics industries can also be drawn on, especially with regard to Scottish research looking at modifying the wavelengths of sunlight before they interact with the solar cell, i.e. improving the match of the incident solar spectrum to better suit the performance of today’s solar cells.

All these developments and others including novel applications such as solar powered water treatment technologies or integrating solar cells into the fabric of buildings is all part of the research mix being undertaken across Scottish universities using multi discipline groups mainly under the auspices of the Scottish Institute for Solar Energy Research (SISER). SISER aims to enhance collaboration in the area of solar technologies-both within
Scotland and internationally- as well as providing expertise and facilities to support up and- coming R&D and future commercialisation in the field.