1. Introduction

Solar energy is the most abundantly available source of energy theoretically. It can be used to produce electricity by thermodynamic process and by photovoltaics conversion [1]. Initially, photovoltaic technology was incepted to convert solar energy into electricity. In this technology, solar cells produce electricity when exposed to irradiance. The amount of voltage produced per unit cell is very small; hence, hundreds of such cells are connected in series and parallel junctions to finally produce high voltage for daily use applications. Such systems are called PV arrays which are available from domestic scale to grid-connected commercial plants. In some cases, solar irradiance is concentrated at a small PV surface to generate more electricity from small PV surface; however, such systems have certain implications like accelerated thermal degradation [2]. Such systems are called concentrated

#### Figure 1.

A novel compact design of heliostat field type CSP with integrated latent heat storage; a) overall schematic and b) close-up view of the heat storage unit and Stirling engine [5].

photovoltaics (CPV). In further advancements, PV cells are integrated onto the buildings to produce electricity by decentralized means [3].

Concentrated solar power (CSP) is another technology to generate electricity from solar energy; however, it works on the principles of thermal energy. Solar radiations are concentrated at a point from where thermal fluid is passed. This fluid at high temperature and high pressure is passed through the power-generating unit (turbine or engine) to produce electricity [4]. The process of CSP plant is illustrated in Figure 1 [5]. The components of a CSP plant are shown including reflectors, receiver, thermal fluid, and turbine/engines. Below is the comparison of photovoltaics with concentrated solar power. Figure 1 shows a recent design of heliostat type CSP plant with a latent heat storage system coupled with the power generation unit. In this design, solar radiations are concentrated using heliostat mirrors on a point at the bottom of thermal energy storage tank containing phase change material (PCM), namely, aperture [5]. The tank is equipped with vertically arranged sodium pipes so that heat can be supplied passively to the entire part of the tank from receiving point. Above the tank is a thermal valve at the interface of sodium pipes. Sodium in vapor phase (at very high temperature) rises through the thermal valve (red arrows) and strikes at the head of Stirling engine where it passes its thermal energy to the energy, get condensed, and flow downward through the funnel (blue arrows). Opening and closing the thermal valve controls the flow of sodium vapors, and hence thermal energy flows to the power block and in a way regulates the power generation of the plant. The concept is proposed and tested for a plant in the capacity range of 0.1– 1 MWelectric (MWe) [5]. In the case where multiple Stirling engines are needed, all will be integrated at the top of the tower in a parallel arrangement.

#### 2. Comparison of photovoltaics with concentrated solar power

The subsequent section covers the comparison of photovoltaics technology with the concentrated solar power in terms of system energy efficiency and systems sustainability.
