Aug-2022

Decarbonising through advances in heat exchange technology

How moving bed heat exchange technology is driving innovation in thermal energy storage.

Gerald Marinitsch, David Moon and Lowy Gunnewiek
Solex Thermal Science

Article Summary

Fossil fuels have been at the centre of a truly remarkable period of development and growth for the global population. In recent decades, however, the negative environmental impact and increasingly high economic cost of fossil fuel use have powered what many are dubbing the “next energy transition” — or should we say “energy evolution” — as we search for new ways to decarbonise our energy use.

Today, this drive to decarbonise our energy needs can be found everywhere — from discussions about a circular economy to the environmental, social and governance (ESG) strategies of businesses around the world.

Given the energy needs of today’s highly connected and mobile world, a tremendous amount of work and money is being directed toward making decarbonised and renewable energy available to us when we need and want it.

This is leading to a pressing need for more readily available energy storage, from which a variety of different technologies are currently available or being developed, such as batteries, pumped hydro, and green hydrogen.

One of the more notable and promising developments in this arena relates to long-duration thermal energy storage (LD-TES) systems that use solid particles. This technology is garnering significant interest and investment primarily because it is targeting a need for storage systems that can provide energy for a period of 10 hours and more. To date, this has not been realised.

An example of this is taking the thermal energy that can be generated from concentrated solar power (CSP) and transferring it to solid particles so it can be stored and used later when the sun is not shining.

To enable this option for LD-TES, a new generation of moving bed heat exchangers (MBHEs) are also being developed so the thermal energy can be extracted from solid particles and subsequently converted into a useable energy form such as electricity
In this article, we will reflect on how changing energy demands have brought us here, along with the unique challenges associated with storing and recovering thermal energy.

We will also explore recent developments in vertical tube and diffusion-bonded MBHEs that are being used — specifically, in CSP systems that incorporate LD-TES. Included will be a discussion around the background requirements for these types of systems, along with design considerations and challenges for MBHEs when being implemented.

Lastly, we will highlight several examples taken from work currently under way that will help illustrate the potential for these particle-based LD-TES systems in helping us evolve to a more decarbonised energy system.

Changing energy demand
Prior to the Industrial Revolution, electricity played an insignificant role. Rather, chemical energy sources such as biomass or oil were common — and useful in that this energy was in a form that was ‘stored’ and useable whenever needed and wanted. Combustion (oxidisation reaction) of the ‘fuels’ typically liberated this energy for use. Concern about carbon dioxide emissions from combustion did not exist then as it does today.

Since then, we have become accustomed to using diverse types of energy in forms such as combusting fossil fuels, wind energy, and solar energy, which are generally converted to electricity for our use.

Yet, in switching from chemical to renewable energy sources such as solar, we are realising there is a shortfall in being able to provide energy whenever it is needed and wanted. To take full advantage of renewable energy as we work to displace fossil fuels in our drive to decarbonise, it is now necessary to store energy during peak production hours when it exceeds demand — which, subsequently, can be used when demand exceeds production.

With that, there are additional factors to consider. As an example, how long does the energy need to be stored? Will energy stores need to be loaded and unloaded in seconds or minutes? Or are the needs more seasonal, requiring the need to store energy for months? All this dictates storage size in closing the gap between generation and utilisation — and subsequently, the required storage technology.

That is not to mention that, with very few exceptions, stored energy will be used as thermal or electrical energy — the latter of which is also the most difficult energy form to store for long periods of time.

The most common method of storing electrical energy today is in batteries. However, batteries are limited in how much and how long they can store energy due to technical and economic limitations.

Consequently, other technologies are needed to store large amounts of energy for long durations. Such technologies are used as intermediate storage, meaning they store energy in a form that can be used to produce ‘on-demand’ electricity.

Long-duration thermal energy storage
LD-TES has been identified as a critical enabler for the large-scale deployment of renewable energy — in particular, within CSP applications.

Before going any further, it is important to differentiate CSP from more commonly known photovoltaic (PV) technology. PV is the direct conversion of sunlight into electricity using the photovoltaic effect in semiconducting materials to directly generate electricity. Storing the electrical energy from PVs is economically restricted to battery technology, and current battery technology is generally limited to short-duration storage — for example, three to four hours. As such, PV technology is largely limited to supplying a base load of power while the sun is shining. In comparison, CSP can generate thermal energy by concentrating the sunlight on a collection point.

In principle, CSP can generate temperatures greater than 2,000°C. However, for practical applications, receiver temperatures operate in the range of 500°C to 1,000°C.

That thermal energy is then commonly used in a CSP plant to generate electricity via technologies such as steam turbines, Organic Rankine cycles (ORC), or supercritical carbon dioxide (sCO2) cycles. In general, higher operating temperatures are desirable because the overall conversion efficiency from light to heat to electricity is higher.

While a CSP plant can easily provide on-demand base-load electricity while the sun is shining, to fully leverage the capacity of this technology, there are strong drivers to be able to store the thermal energy generated during daylight hours. The objective is to have enough thermal storage so electricity can be produced even when the sun is not shining over a period of days or longer.

Today’s commercial CSP LD-TES plants commonly use molten salt to store the thermal energy collected from the sun. A key limitation of this configuration, though, is the overall conversion efficiency that can be realised due to the operating temperatures practically achieved with molten salts.