ZCA explainer series: solar energy

This explainer outlines the different kinds of solar technology: Concentrated Solar Power (CSP), Solar Photovoltaic (PV), and solar thermal.

Published

October 24, 2023

Concentrated Solar Power (CSP) is one of two technologies that use the sun to generate electricity

Source: Unsplash

Concentrated Solar Power (CSP) is an indirect method of electricity generation. It uses multiple mirrors to reflect the sun’s energy onto a receiver, where it is converted into high-temperature heat. Thermal energy storage systems store the heat produced, allowing it to be used flexibly when there are periods of less sunlight. This makes CSP attractive for large scale power generation. The heat is then turned into steam, which drives a turbine to generate electricity. Alternating current (AC) is generated, which is easy to distribute on the power network.

There are four kinds of CSP technologies:

1. Parabolic trough systems- Curved, trough-shaped reflectors are used to concentrate solar energy onto a receiver pipe that contains thermal oil. The oil is heated by the solar energy and then used in a thermal power block to generate steam.

2. Power tower systems- Heliostats track the sun and reflect its energy onto a receiver on top of a tower. Inside the receiver is a fluid (usually molten salts) that is heated and used to generate steam to drive a turbine generator.

3. Linear fresnel systems- A large number of mirrors are laid out on the ground in rows with a receiver pipe above. Energy can be stored in a power block or steam can be generated directly.

4. Parabolic dish systems- A parabolic-shaped (a plane curve that is u-shaped) dish concentrates and reflects solar energy onto a receiver. The receiver is on a structure that has a sun-tracking system. Heat is then generated by a heat engine.[i]

CSP systems can be combined with other power plants, such as thermal-fired power plants using coal, natural gas, and biofuel, to create hybrid power plants.

CSP production has increased over the last decade, but since 2019 there has been a drop in production, from 14.2 terawatt hours in 2019 to 13.6 in 2021.[ii] Spain had the highest recorded installed CSP capacity in 2022 at 2.3 GW. This was followed by the USA with an installed capacity of 1.5 GW, and China with 596 MW.[iii]

Solar Photovoltaic (PV) is the other solar technology that generates electricity

The first thing that needs to be understood when learning about solar PV is the difference between PV cells and solar panels. PV cells are a component of a solar panel. They are usually made of monocrystalline or polycrystalline materials and tend to be square in shape. A collection of multiple PV cells in a frame is a solar panel or photovoltaic module. Solar panels for home installation are usually made of either 60 or 72 solar cells.[iv] A solar array is a collection of multiple solar panels. PV cells only capture about 11-15% of the sunlight’s energy, so, a large number of PV cells are needed in a solar array to make up for this.[v]

PV solar panels convert sunlight (rather than thermal energy) directly into electricity. Solar PV cells have a higher number of electrons on their front, creating a negative charge, and fewer electrons on their back, creating a positive charge. When the PV cells absorb light, the electrons are knocked loose and they circle the cell, creating voltage and current. When the current is forced into wires, a direct current (DC) is generated. It can then be converted into AC using inverters, and from there it can be distributed on the power network.

PV systems cannot store thermal energy like CSP can because they generate electricity directly. Historically, storing electricity from PVs, particularly at large power levels, has been a challenge. Fortunately, battery storage is a rapidly evolving technology.

Over the last decade, the price of a completed PV system has dropped by 59%.[vi] This trend is expected to continue for some time. Fraunhofer states that in Germany, prices for PV power plants have fallen by over 75% since 2006 due to learning effects and economies of scale.[vii] Between 2010-2020, the PV technology market had a Compound Annual Growth Rate (CAGR) of 34%.[viii] In 2021, there were 132.8 GW of solar PV capacity installations, compared to 125.6 GW in 2020. [ix] China accounted for 40% of the 2021 additions. As a result of the rapid growth of solar PV, employment in this sector is also growing the fastest out of any renewable. In 2021, over a third of the total renewable energy workforce worked in solar PV.[x]

Increasing demand for renewable energy, including solar, is creating the challenge of competition over land and associated acquisition costs. One possible solution is floating photovoltaics (FPV). There is a body of research that is investigating the electricity output that could be generated by installing FPVs on hydropower reservoirs. A study by Kakoulaki et al. (2023) shows that there are 337 hydropower reservoirs in the EU27, with a total area of around 6,086 km². If 1% of this area was used for floating solar arrays, 13.87 TWh per year could be generated. If 10% or 100% of this area was used for FPVs, 138.67 and 1,386.7 TWh/yr could be generated respectively. In the 100% area scenario, 50% of the EU27’s total electricity generation could come from installing FPVs.[xi]

Solar power is more than just electricity

Solar thermal or solar water heating is a way of generating hot water or space heating for a home or business. Solar thermal systems can even be used to heat swimming pools. Thermal water collectors will usually be installed on the roof of a property. In a domestic property, what usually happens is that there are fluids inside the collector tubes that heat up from the energy of the sun. The fluid then flows to a heat exchanger inside a water tank. Thermal convection exchanges the heat from the fluid to the water in the tank. The fluid then circulates back into the collector tubes. The hot water can supply washing machines, bathrooms, dishwashers, sinks, underfloor heating, and radiators.

There are two main types of solar thermal systems. Active solar water heaters use electric pumps, valves, and controllers to circulate heat transfer fluids, whilst passive solar water heaters do not use pumps to circulate this liquid. The lack of electrical components makes passive systems cheaper, more reliable, and easier to maintain.[xii]