Concrete is the world’s second most utilized material after water. Each year, about one cubic metre of concrete is produced for every man, woman and child on the planet.
In and of itself, concrete is a durable and environmentally friendly material. Unfortunately, the kilns that produce Portland cement, a key constituent of concrete, also produce large amounts of carbon dioxide—roughly one ton of CO2 for each ton of cement. It is estimated that up to 9% of all anthropogenic CO2 emissions come from cement production.
Cement is also a key constituent of HEATCRETE®, the solid-state storage medium used in EnergyNest’s thermal energy storage (TES). It should come as no surprise that one of the main concerns expressed regarding the EnergyNest TES is the carbon footprint associated with the cement production required for its HEATCRETE® storage medium.
So how is it that EnergyNest turns cement from a carbon emitter to a carbon reducer?
CO2 debt and carbon payback
A good way to measure the effectiveness of a technology in offsetting carbon emissions is to calculate its carbon payback time. The exact carbon savings achieved through the TES will depend on a number of factors, including the emissions linked to the construction of the storage (its carbon “debt”), and the type of fuel consumption that the stored energy will be offsetting.
In order to store 1 MWh of thermal energy, one typically needs around 40 tons of HEATCRETE®, which contains about 5.5 tons of cement. The carbon emissions associated with 5.5 tons of cement production is roughly 7 tons of CO2.
If one assumes that the EnergyNest TES is operated with one cycle per day, then all one has to do is to divide the CO2 for the cement used in the TES by the equivalent CO2 emitted from saving 1 MWh of fossil fuel. When substituting coal, which emits around 370 kg of CO2 per MWh, this gives us a carbon payback period of 19 days.
The EnergyNest TES is made of extremely durable components and has a very long lifetime. Although generally projected over 20 years, it can be recommissioned for further use, up to 50 years of operation in total. This means about 18 000 days of operation over its lifetime. In our case, each day the TES would store and discharge 1 MWh of thermal energy, meaning that the TES would discharge 18 GWh of thermal energy over its lifetime.
When we then calculate the cement-based carbon content of the energy discharged from the EnergyNest TES over its lifetime, we obtain no more than 0.4 g/kWh. Compare this to the emissions from burning coal, which are more than 370 g/kWh! The potential for carbon emissions savings is substantial.
The numbers above only consider the emissions linked to cement production. A full assessment may also calculate other emissions arising from fabrication (steel smelting, mining of aggregates, electrical and mechanical components), as well as emissions arising from construction (transportation of components, building foundations, commissioning).
All of these emissions are highly dependent on context and likely to vary from one project to another. What is certain is that their effect will generally not be substantially more significant than that of cement. A conservative total estimate would put overall emissions at 15 kg per kWh of storage capacity. Comparing this number with about 300 g CO2 per kWh from burning fossil fuel, one can see that a reasonable number for payback of the CO2 debt for a TES is less than 2 months.
Therefore, no matter what type of project, the EnergyNest TES will recover its carbon debt very quickly. The EnergyNest TES thus constitutes an elegant and cost-effective solution for reducing carbon emissions in heat-intensive industries and electric power generation, in addition to providing a positive way for the cement industry to offset CO2 from their overall cement production.