The human race has a long history of harvesting solar energy. We had been using the thermal energy from the sun to dry our clothes and food since early civilisation.
We all enjoy the warmth under the sun in the cold weather, it is therefore natural for the human race to utilise the solar energy in the form of thermal energy. The first type of widely applied solar energy system was understandably the solar thermal system. Generally, it absorbs the heat from the sun to heat up a media of thermal storage for further use (e.g. heat up water for domestic application). Other more sophisticated applications include concentrating the sunlight to a solar tower to collect the heat for generation of electricity. These complex systems are still developing and not widely applied up till now. Nevertheless, the domestic use of solar thermal systems is already a commonplace application nowadays. Figure 5.1 shows a domestic solar thermal system for preheating the water to supplement hot water supply in a residential building.
1Department of Electrical and Electronic Engineering, The University of Hong Kong, Hong Kong, China
Although harvesting the thermal energy from the sun is a convenient and economical way, it would require another step to convert it to the common type of energy carrier that modern people are enjoying – electricity. Therefore, the most widely applied type of solar thermal system is to utilise the thermal energy for heating up the domestic water. On the other hand, electricity is also the specific energy carrier that could link up solar energy and EV without the need of further energy conversion. Hence, this chapter will be devoted to the technology of har- vesting solar energy directly by transforming sunlight into electricity. This type of technology is called PV, which will be explained in more details in the following section, followed by two case studies illustrating how this technology could be applied to vehicles.
5.1.1 Brief history and types of PV technology
PV technology – the direct conversion of electricity from sunlight – was rooted from the discovery of photogalvanic effect by Becquerel in 1839 (Messenger and Venture, 2000). The first PV cell, however, was only successfully materialised in 1954 with the advancement of material science and manufacturing of solid state devices. It is now widely accepted as a benign technology to generate electricity from sunlight with minimal pollution (Martin, 1997), and as one of the possible alternatives to fossil fuel energy sources. Kurokawa (2001) showed that PV had the merits of simplicity as modular energy source which can be easily deployed and installed in the urban environment.
There are many ways to apply PV for harvesting the solar energy. The applica- tion with the longest history was to power the energy requirements of satellites at such a location where no other means of reliable and maintenance-free power source could be possible. With the advancement in manufacturing of semi-conducting devices Figure 5.1 A domestic solar thermal system installed on the roof of a house in
Hong Kong (Photo courtesy: Mr. Jackson FUNG)
which mainly consist of silicon, the development of PV cells from the by-products of semi-conductor industry could lower the cost and made civil use of PV feasible (Hegedus and Luque, 2003).
Throughout the history of development of the PV technology, its dominating applications have long been the stand-alone applications. In a stand-alone PV system, PV is the only source of energy powering all the electrical loads by itself.
To cater for the demand when there is no sunlight, it requires some kind of energy storage: by means of battery, water tanks, fuel cells, etc. These types of systems are widely used for the rural and remote applications that required modest power or where the supply from utility was not easily available. An EV running on the road, of course, can be a good candidate of getting at least part of the electricity supply if PV modules are mounted on its body. Only towards the end of last century (International Energy Agency PVPS T1-23, 2013), these stand-alone PV systems fell below 50% of total worldwide PV installations. The percentage of stand-alone systems went down rapidly to below 20% by the end of 2004 (ditto). Nowadays, the dominating type of PV application is the grid-connected PV system producing clean power for contributing to part of the electricity grid.
The operational principle of grid-connecting PV system is to convert the direct current (DC) electricity from the PV modules to alternating current (AC) with sophisticated power electronic devices and then supply the power to the electrical loads in conjunction with the utility grid. It serves as a supplementary power source to main-stream generation with fossil fuel, nuclear or other conventional means from the utility. Effectively, the grid-connecting PV systems use the grid power as the storage and also the complementary source. It would supply the loads together with the grid when there is not enough power from the sun, and would generate excess power into the grid when the PV system could provide more than the loads is needed. This is the most fast-increasing type of PV application and became the dominant type by the end of 1999. Lutheret al.(2003) argued that this type of PV system will be contributing to the main-stream power production for the reduction of CO2emission in industrialised countries. Hence, the course of change in PV application indicates a development of its role in supplying power to remote areas where no other means of electricity could be easily available and then to supporting the modern society in reducing the adverse effect of burning fossil fuels. Table 5.1 summarises the history of development of PV (Hegedus and Luque, 2003). It demonstrates that alongside the beginning of the new century, PV industry is entering the era of grid-connected applications.
Although grid-connecting PV systems are now the dominating type, they can only help in powering the EVs when the EV charging station has such systems connected to it. The more direct way to utilise solar energy is to install PV cells onto the body of the EV for providing all or part of the energy needed. This will be explained more in the next section.
5.1.2 Harvesting solar energy for EVs
When an EV is running on the road during daytime, it is constantly receiving radiant energy from the sun unless there is thick clouds covering the sky. Under Solar energy harvesting for electric vehicles 131
normal situation, part of that radiant energy would be converted into heat that would raise the internal temperature of the vehicle (including the temperature of the electronic circuits and the battery bank), hence decreasing the overall efficiency of the vehicle. There were different trials of mounting PV cells onto the body of the vehicle to provide part of the electricity required by the EV. To promote the research and development (R&D) activities for such applications, a team of researchers is organising a semi-annual event called the World Solar Challenge (http://www.worldsolarchallenge.org/). The race requires the EVs to run 3,000 km all the way from Darwin to Adelaide, Australia; and the only source of energy is from the sun. The vehicles taking part in the competition demonstrate the extreme of the possibility of harvesting solar energy for EVs. Every vehicle would need high efficiency PV cells, sophisticated power management system (PMS) and effective battery bank to enable it to run over 3,000 km. Nevertheless, these vehi- cles are setting a high and yet reachable goals for our future research into this particular area. In the official website of World Solar Challenge, they describe the participating EVs being ‘arguably the most efficient electric vehicles’. Figure 5.2 shows the champion car in the World Solar Challenge 2013.
In order to appreciate the challenges we are facing if we intend to power the entire EV by PV cells, we have to understand the operating principles, conversion efficiencies and limitations of the PV cells when they operate in the actual outdoor environment. The following section will then outline these background information.