Three quarters of GHG emissions come from the energy sector. However, currently our efforts are more towards electricity generation and road transportation. Another form of energy, heat, is often overlooked. Chemicals, food, metals, and most manufacturing industries require large amounts of heat, therefore, a huge carbon emissions! Is there a way to decarbonize our heat demand?
We need heat for everything
From industry to household, making steel to making your meal, heat is needed for all kinds of processes in all sectors. In 2018, it accounts for 50% of global final energy consumption and emits 40% of global carbon dioxide (CO2) emissions¹.
Half of that heat is used for industrial processes. Just to mention:
- Food and beverage: boiling, drying, pasteurizing, rendering, sterilizing, and washing.
- Textile: bleaching, drying, dyeing, and washing
- Chemical: boiling, distilling, and reaction processes
- Paper: bleaching and drying
As you may guess, most of the heat comes from burning fossil fuel. Heat can be generated from renewable resource, such as biomass. But biomass is not always easy, especially if the source is not available nearby, not to mention the way of the biomass is produced, is not necessarily sustainable. Therefore, decarbonizing our heating need would require a dramatic shift in how heat is generated.
Boiler, Centuries Old Technology
Steam is the most popular medium to transfer heat. To transfer heat from burning fuel to water, we need a boiler, a device that started the industrial revolution. Yes, our way of generating steam doesn’t change for centuries, boilers just get more efficient while keep using the same kind of fuel.
Boilers are available in various sizes and operated in multiple industries.
Solar Thermal, Beyond Hot Water for Shower
Solar PV is not the only way to capture sunlight. Quite straight forward, solar thermal technology converts sunlight into heat. It receives direct sun irradiation, concentrates the light through a set of mirrors which heats up fluids, such as oil and molten salt. You might think that heat from the sun is just enough to heat water for shower. It is true when we are not concentrating the sunlight. With concentration (hence its name, Concentrating Solar Power), temperature up to 1000 degrees Celcius can be reached! Then, what’s stopping us from utilizing the sun to replace the fossil-fueled boiler?
Where Does Solar Thermal Beat Natural Gas?
Parabolic Trough is a mature technology, it could reach temperature up to 400 degrees Celcius. Inside the absorber tube (see image), heat transfer fluid, typically oil, is circulated and pumped to a heat exchanger, where heat is transferred to produce steam.
We have the technology which potentially replaces fossil fuel in producing steam. Let’s consider natural gas as currently is the main fuel choice for small to medium boilers. Now, the question is, at what price of natural gas where installing CSP is more beneficial? This question is even more interesting as the sun intensity varies on geographic location and local climate.
On the left, I plot countries based on how much solar intensity it receives (Direct Normal Irradiation) on the X axis and how much the natural gas price on the Y axis. The circle size indicates the country’s annual natural gas consumption while the color shows whether it is beneficial to switch from natural gas to solar for the same amount of steam produced. I conclude, even for countries with low solar resources (about 1000 kWh/m2), natural gas price at the level of 10 USD/MMBTU is enough to make profit by switching to solar thermal boiler. However, my calculation underestimates the total cost of system installation. Absolicon, solar collector manufacturer, provides cost simulation of 93 USD/MWh steam, which is equal to 27 USD/MMBTU natural gas price³. Moreover, the natural gas price I put on the plot most likely to be inaccurate or outdated, but you can match your own factory natural gas price you pay from the provider.
Then you might ask, if those are true, most boilers should have already been replaced by solar thermal, right? Well, free energy from the sun doesn’t come without drawbacks.
Sunlight is a dilute form of energy. For every one kilogram of fuel, coal and natural gas contains 24 MJ and 50 MJ of energy, respectively, while one square meter of area receives only 20 MJ of solar energy in one day. In practice, to generate one ton per hour of steam, one must dedicate 5500 sqm of land (with parabolic trough). In comparison, a boiler with similar capacity would require an area as small as your bedroom.
What about intermittency? Guess what, we have mature technologies to store heat! Thermal energy storage (TES) allows excess heat to be stored and used hours, days even months later. It comes in widely differing technologies, one example is molten salt which stays liquid at 290 degree Celcius⁴.
Can we put it on the roof? The solar collector is a delicate tracking mechanism, demanding optical precision, thus rigid support is imperative. Moreover, mirror deforms due to gravity, wind, and thermal expansion while surface imperfection results in defocussing. Those technical difficulties prevent development of small solar collectors as solar PV would easily be put on houses’ roofs⁵. One company, Sopogy, is trying to develop MicroCSP that can be put on rooftops⁶.
Solar thermal offers an alternative solution to decarbonize industrial process heating. With current technology, it is already at parity with natural gas price. A major drawback is the large area needed which won’t be available in several industrial sites. My recommendation is utilizing this technology as an additional heat source rather than complete replacement in any heat intensive industry with available unused space.
Following are several values I use for the calculation, which I collected from these great publication:
Trieb, Franz, et al. “Global potential of concentrating solar power.” German Aerospace Centre (DLR) (2009).
Dieckmann, Simon, et al. “LCOE reduction potential of parabolic trough and solar tower CSP technology until 2025.” AIP Conference Proceedings. Vol. 1850. №1. AIP Publishing LLC, 2017.