Solar Electrical Energy Storage

Yulong Ding1; Yongliang Li1; Chuanping Liu2; Ze Sun3    1 School of Chemical Engineering & Birmingham Centre of Energy Storage, University of Birmingham, Birmingham, UK
2 Department of Thermal Engineering, University of Science and Technology Beijing, Beijing, China
3 National Engineering Research Centre for Integrated Utilization of Salt Lake Resources, East China University of Science and Technology, Shanghai, China

Chapter Outline

2.1 Background

Modern societies become increasingly dependent on reliable and secure supplies of electricity to underpin economic growth and community prosperity. This makes electricity an important vector in current and future energy systems, with the latter particularly related to electrification of heat and transportation. In the United Kingdom, the current end user demand on electricity accounts for around 18%. Under the Carbon Plan scenarios, this share will increase to 25-31% by 2030 and 33-44% by 2050 (Taylor et al., 2013). Globally, the net electricity generation will increase by 93% in the IEO2013 reference case, from 20.2 trillion kWh in 2010 to 39.0 trillion kWh in 2040 (EIA, 2013). This implies that the world electricity generation will have to rise by 2.2% per year from 2010 to 2040, compared with an average growth of 1.4% per year for all delivered energy sources. Electricity will supply an increased share of the world's total energy demand, and hence become the world's fastest growing form of delivered energy.

Currently, electricity is produced mainly from fossil fuels. However, due to the long-term pernicious effects of greenhouse gas emissions on the environment, the decreased availability of fossil fuel resources, and the growing sense of urgency toward energy security, the use of more and more renewable and environmentally sustainable energy resources is inevitably happening and is expected to be dominant in the foreseeable future. Solar energy is regarded as a leading contender for green energy production. In fact, solar power installations are currently increasing by 40% per year worldwide (Ginley et al., 2008). According to the estimation of Energy Technology Perspectives 2014, solar power could be the dominant source of green energy by 2050 (IEA, 2014).

Solar energy can be converted to electrical energy in two main ways (Li et al., 2012). One is through solar cells (photovoltaic technology), which directly convert the short wave range of solar radiation energy into electrical energy. The other is via an indirect solar thermal route, which converts the solar radiation energy into thermal energy by means of solar collectors or concentrators, which then generates electricity through a conventional thermal cycle. However, sunlight is diffuse and intermittent. Weather conditions also determine the availability; power generation using both the technologies is unpredictable and unreliable. Therefore, substantial use of solar power to meet humanity's needs requires electrical energy storage to ensure a reliable power supply.

2.2 Technical Requirements of a Solar Electrical Energy Storage Facility

Currently, solar cells and solar thermal power systems cover a wide range of applications, from less than 1 W to 100s MW, as shown in Figure 2.1 (Quaschning and Muriel, 2001). It should be noted that solar thermal power plants can only use direct solar irradiance for power generation, while solar cells can convert both direct irradiance and diffuse irradiance. Therefore, solar cells can produce some electricity even with cloud-covered skies, making them applicable even with very low solar irradiation. Generally speaking, solar cells are most suitable for small-scale low-power demands, which are able to operate as standalone systems as well as grid-connected systems, whereas a solar thermal power plant is often a better option for large-scale and grid-connected systems. Due to different applications, there are different configurations to create a solar electricity installation with a solar cell facility, a solar thermal power plant, or both. However, from the role in the electricity supply chain, the installation can be classified into two categories: utility-scale solar electrical facility and distributed solar electrical facility.

Utility-scale solar power generation refers to medium- to large-scale solar energy installations, which can either be thermal power plants or solar cells. These units are designed to generate large amounts of electricity, which require large vacant lands and therefore are located in rural or semi-wild regions. As a result, they are far from end users and have to be physically connected to existing grids at discrete points. With increasing solar electricity penetration, utility-scale energy storage systems are required to provide utility-controlled functions, including long-duration electricity shift and capacity firming. Although there are no recognized standards at present, it is expected that the storage systems should have a maximum power rating of 1-20 MW (charging and discharging) and the ability to store 2-6 h of energy for on-demand delivery to the electric grid (EPRI, 2011). With such capacity, the storage system can provide a tremendous advantage to solar power generation efficiency and production, while lessening the negative effects of solar power generation on the grid. The energy storage systems are also expected to be used as a spinning reserve to delay committing additional fossil fuel power generation units. These imply that the following are required for energy storage systems for utility-scale solar power generation:

• Storage properties—high storage capacity, long charge/discharge times, good partial-load feature, and acceptable round-trip efficiency

• Financial performance—low capital cost, easy to maintain, and environment-friendly

• Other aspects—fast start-up and response for load following

On the other hand, distributed solar power generation refers to small- to medium-scale systems. Such systems are most commonly solar cell based, except for dish/Stirling solar thermal power systems. They are designed to generate moderate amounts of electricity, which require a small amount of land; hence, they can be placed in local electrical distribution systems at both the generation and use points. They could either be stand-alone systems, or they could be used to generate more electrical energy in conjunction with nearby installations. Compared to utility-scale systems, the generated electricity from distributed systems can only be used...