Solar Thermal Energy Storage Systems and Their Role in Electrifying our Energy System
Updated: Nov 24, 2020
(Photo Credits: OurFuture.Energy)
It has been reported that a lot of people on LinkedIn and also in energy media coverage industry consider "Electricity" and "Energy" quite interchangeable. ... Which is far from truth. Although Electricity is a very important part of the world's energy system, but it's far from the only part. Electricity makes up only a quarter of our total energy consumption. It is because most of the energy intensive sectors (i.e. heating, transportation, building, some heavy industry, aviation, shipping, agriculture etc.) are still not ready to be electrified directly or indirectly cost effectively by these renewable technologies (i.e. solar PV, onshore wind, offshore wind etc.) and still get their energy from fossil fuels. This is, because these renewable technologies (i.e. solar PV, onshore wind, offshore wind etc.) need battery storage technologies to store the electricity during the off peaks hours in order to supply power for the peak hour demand and although battery storage technologies have improved a lot but still none of these systems can store large amounts of electricity at reasonable costs or efficiencies.
According to BloombergNEF, a research company, direct and indirect electrification of transport, buildings and industry could deliver a 60% reduction in carbon emissions across Europe by 2050. Therefore, it is important to explore and implement different approaches to electrification of our energy system.
This article particularly focuses on the role of Solar Thermal combined with thermal Energy Storage Systems for a significant portion of electricity generation.
There are two main methods of converting sunlight’s photons into useful energy: solar photovoltaic technology (PV) and concentrated solar power (CSP). Solar photovoltaic technology (PV) converts sunlight’s photons directly into electricity with the help of semiconducting materials (i.e. silicon) that exhibit photovoltaic effect whereas concentrated solar power (CSP) technology uses mirrors to gather heat from a large area to a small area and then utilizes this solar heat to raise the temperature of heating fluid (usually molten salts) upto 550 ᐤC which can be used either for conventional steam cycle or can be stored for later dark hours. In this way, it can overcome the shortcomings of solar PV enabling the grid to provide electricity during night time as well.
Fig 1: General schematic of a concentrated solar thermal plant
There are four types of solar concentrators designs available in the market but the basic principle always remains the same; concentrating the solar heat using mirrors onto a smaller focused area to heat up the fluid which can be either oil or molten salt. The fluid is then sent to a heat exchanger where the second fluid, which is usually water is converted into the steam and then sent to the steam turbine for conventional electricity production cycle.
Following types of solar concentrators are already available in the market (Fig 2).
- Solar towers - Parabolic troughs - Linear Fresnel reflectors (LFR) - Dishes
Fig 2: Types of solar concentrators
Thermal energy storage (TES) has been proved an important sub-system for the solar energy generation systems (SEGSs) because it considerably increased the SEGS performance. This distinguishes the solar thermal power plants from the conventional fossil fuel thermal power plants. A well designed, operated and managed molten salt TES system for SEGS achieves several goals: (1) To release energy for electricity generation after sunset for several hours to meet the power consumption peak without the fossil fuel backup, and make the SEGS more independent of the weather fluctuations over time so that it increases the efficient annual usage of sunlight. (2) To generate higher temperature steam, e.g. over 450 ᐤC, for turbines so that it raises the Rankine cycle efficiency up to 40%. In comparison, the expensive high-temperature oils generate steam around 390 ᐤC, which gives only 37% Rankine cycle efficiency, as shown in Fig. 3.
Fig 3: Comparison of Rankine cycle efficiency using different heating fluids.
(3) To improve the system performance and economic index, such as reduce the levelized electricity cost, for the SEGS due to the above two points.
(4) To provide clean energy as this process involves no chemical reaction or combustion and only thermal energy is converted into electricity by steam cycle.
(5) To provide flexibility to the energy system by not only providing electricity but also a reliable source of thermal energy in energy intensive sectors.
As reported by BloombergNEF, a research company, electrifying our world's energy system can definitely accelerate our path towards decarbonization. Therefore, it is important to explore and implement different approaches to the electrification of our energy system in order to achieve deep carbon emissions cuts.
Even though solar thermal energy currently doesn’t represent significant fraction of electricity generation, therefore, utilities do not care when the electricity is being generated. However, if solar thermal power plants began to represent a significant portion of electricity generation, then the value of baseload solar thermal energy will likely increase and molten salt storage systems may become essential reduction in carbon emissions.
The recent successes of solar PV, wind, batteries and electric vehicles have shown that policy and technology innovation have the power to build global clean energy industries. In my opinion, Solar Thermal Energy Storage Systems can play an important role in the development of sustainable energy ecosystem of the future, if national and local governments, industry and investors support these systems and work together to scale up technologies and bring down costs it can enable the faster electrification of our energy system and achieve deep carbon emissions cuts.
Abdul salam is an enthusiastic, InnoEnergy Master graduate in Energy engineering, who is currently working on Portugal's first Molten salt thermal storage project named NEWSOL, where he works on the project technical design, planning and working procedures.
Qin, Frank & Yang, Xiaoping & Ding, Zhan & Zuo, Yuanzhi & Shao, Youyan & Jiang, Runhua & Yang, Xiaoxi. (2012). Thermocline stability criterions in single-tanks of molten salt thermal energy storage. Applied Energy. 97. 816–821. 10.1016/j.apenergy.2012.02.048.
Stojicevic, Misa & Jeli, Zorana & Obradovic, Milos & Obradovic, Ratko & Marinescu, Gabriel. (2019). Designs of Solar Concentrators. FME Transactions. 47. 273-278. 10.5937/fmet1902273S.