The aim is to reduce the industry energy bill providing a cost-effective solution that make possible the recovery of large amount of waste energy to be used on demand. Industry energy bill savings may reach up to 30% while contributing to reach Europe energy efficiency targets.

It is simple, low-cost (5-10 €/kWh), maintenance-free and easy to operate. This technology is based on a packed bed arrange of low-cost solid particles, where to carry out the charging and discharging operations of the storage, the heat is exchanged directly between the waste heat stream and the solid particles as sensible heat, increasing or decreasing the temperature of the solid particles. Both the development of an appropriate solid filler material and the optimal design of the system by fluidynamic modelling are the secret.

More than 10 years ago, CIC energiGUNE initiated an intensive research activity in thermal energy storage for industrial heat processes to make the recovery of industrial waste heat a competitive solution. The activity comprised a deep research on materials, fluid-dynamic modelling and engineering, within several European and industrial projects such as ORC-Plus and ReSlag. This work led to the successful commissioning of several real-scale prototypes carried out under the scope of the CIC energiGUNE’s systems engineering group, such as the off-gas waste heat recovery unit commissioned in the world’s leading steel company ArcelorMittal.

Industrial waste heat recovery or recovery of the industrial environmental concern

The necessity of determined actions against the global warming is unquestionable and process industry may play an important role.

The industry sector is responsible of more than one third of the total energy consumption worldwide, with an increasing trend mainly driven by the slight growth of the energy intensive industries (EIIs). EIIs are those industries with the largest carbon footprint and are often represented by: food, pulp and paper, basic chemicals, refining, iron and steel, nonferrous metals (mainly aluminum), and nonmetallic minerals (mainly cement).

The main type of energy end-use among these industries is heat, accounting for 50% of the total worldwide heat consumption, which represents by far the largest energy end-use type.

On the supply side, nowadays heat production means are clearly dominated by fossil fuels, but worst of all, is that once the heat finishes its mission in an industrial process, somewhere between 20 to 50% of the energy input is lost as waste heat. Hot exhaust gases, cooling water streams or free heat losses from incandescent surfaces are widespread examples of a free-of-charge contribution to increase the exergy of the universe.

Furthermore, most of the times, even more energy is fed into the process to cool down the waste hot source, since the temperature is too high to be released directly to the atmosphere.

One may easily conclude that industrial heat use is the least and worst efficient way of using energy, but it would be a not very precise conclusion. The inefficiencies belong to a whimsical scenario where burning fossil fuels still became cheaper than recovering and reusing the excess of the so-called waste heat.

Nevertheless, the rules of the game are changing fast, primarily represented by the progress towards phasing out fossil fuels subsidies, whilst energy efficiency subsidies keep increasing.

The contribution of these policies, in addition to the technology development, are reaching to a point where the return of investment of a solution to recover and reuse waste heat may claim the interest of the financial managers, since it is becoming a reliable investment.

Economic analysis of a real case in the steelmaking industry

Considering the average electricity price value in Europe at present (0.125 €/kWh), the current state-of-the art in waste heat-to-electricity solutions within the steel making industry may ensure an average return of investment of around 4 years, which may still decrease in the coming years.

In order to support this conclusion, we present in detail the analysis of a real case.

Within the steel industry, the electric arc furnace (EAF) has experienced a significant growth in the modern steelworks concept and thus, it will be considered as the reference scenario.

The energy balance of a typical EAF implies consuming 700 kWh per ton of produced steel, out of which, almost 150-250 kWh per ton of steel is identified in the EAF manuals as the total sensible heat energy released at the EAF outlet as hot exhaust gases.

If due to interferences on the steel production process, the off-gas stream cannot be extracted right at the exit of the EAF (which may often be the case), a more conservative and realistic scenario would consider that the usable energy content of the off-gas stream at the next available extracting point may decrease down to around 100 kWh/t.

The casting process in an EAF involves around 40-45 min of scrap melting and 15-20 min for process handling. That means that the waste heat is only available for 45 min during an hour.

Considering a steelwork in a continuous operation mode during a whole year, operating 1 EAF with an average production capacity of 100 t/casting, the total waste heat production per year accounts for 65,7 GWh, as thermal energy. Taking into consideration that the efficiencies of commercial heat-to-electricity systems (often Organic Rankine Cycles) fall in the range of 10-20%, the corresponding electrical output may reach 9,8 GWh per year, with an electrical power of 1,5 MW.

According to data provided by the LIFE HRII project, the CAPEX and OPEX cost of the required whole waste-to-electricity plant for this particular case, without considering any storage, would reach around 4 M€ and 200 k€/year, respectively.

The incomes of the case study are represented by the plant electricity production and the corresponding savings in the electricity bill. The electricity prices for non-household consumers have remained stable in Europe over the past 10 years increasing similarly to the overall inflation, but the weight of the taxes has increased continuously from 13.8 % in the first half of 2008 to 35.3 % in the first half of 2020, turning into an overall significant increase.

The average electricity price evolution in Europe for non-household consumers is showed in the following figure, representing a very promising scenario for investment in electricity production facilities. Taking into consideration the data provided above, we include below a simple approach to the ROI calculation of the reference case study in the Spanish market and in the European average market (Electricity prices fixed to 2020 value).

As can be inferred from the graph, the ROI results may vary between less than 4 and 5 years and therefore, taking into account that the electricity prices will keep increasing, it becomes a very interesting investment.

It shall be highlighted that this approach represents a very conservative scenario, provided that none of the multiple subsidies available within european countries for energy efficiency improvement actions or the benefits concerning an increase disposal of emission allowances, have been considered in the calculations.

The role of the thermal storage

There is a general belief that a TES system is required only to decouple the heat source from the heat sinks, especially suitable for batch processes. Nevertheless, the most relevant effect of including a TES system in a waste heat recovery solution, is ensuring a reliable return-of-investment (ROI) calculation of a waste heat recovery system to potential investors, which otherwise may present important uncertainties.

Anyone working in the industry, and specially in EIIs, will agree that the idle production conditions are something that one has never seen during a whole professional career. From production disturbances due to machine failures, planning unexpected changes to maintenance activities, the production is never maintained continuously for 24 hours, 7 days a week. In addition, many industrial activities are already designed as batch processes with inherent discontinuous operations.

As a practical example, if we consider the particular case of the off-gas stream produced in the electric arc furnaces (EAF) within the steelworks, a non-uniform hot stream is produced only during the casting time lasting approximately 40-45 min, during the rest 15-20 min of an hour, there is a significant decrease on the energy that can be extracted from the EAF off-gas and delivered to a potential heat-to-power device, to produce electricity.

The resulting consequence of these non-uniform streams and intermittencies force to either oversize the plant, increasing the investment cost, or to loss plant efficiency due to continuous partial loads or switching on/off operations. In this case, the ORC efficiency given by the ORC provider and used to estimate the ROI may never be reached, so it must be taken into serious consideration if anyone is considering such an investment project.

As can be observed in the following figures, these effects have a significant impact on the financial viability of the technology. Considering data provided above regarding investment costs and the Spanish market as a reference scenario, an ORC efficiency drop from 15% (purple line) to 10% (blue line) would almost double the ROI time of a waste heat recovery solution. On the other side, ORC turbomachinery efficiency is very sensitive to partial loads, and this efficiency loss of 5% may easily happen when the load decreases below 50%, which in the case of the EAF within the steelworks happens every hour during 15-20 min.

Nevertheless, the solution to this inherent weakness of the batch or discontinuous processes is much closer and simpler than one can imagine but probably it is still facing lack of dissemination issues.

Including a TES device in a waste heat recovery solution to deal with the intermittence and non-uniform temperature nature of industrial waste heat is a well-known approach and the technology developed by CIC energiGUNE has been designed to fulfil the industry needs.

Firstly, the positive effect of the TES technology developed by CIC energiGUNE on the waste heat recovery plant may be observed in the following pictures, applying the solution to a particular EAF off-gas temperature output profile at a given potential extraction point. Collecting the off-gas stream through an adequate TES system, the non-uniform temperature profile may turn to an almost isothermal stream during the whole EAF operation cycle, ensuring high and constant ORC efficiencies and therefore, providing reliable data to assess the viability of the waste heat recovery solution.

Finally, the research activity of CIC energiGUNE has turned into customizable TES technology with optimal performance and capital cost in the range of 5-10 €/kWh.

The following picture shows the insignificant effect of the TES solution cost in the overall IWHR plant ROI calculation, which barely increases in two months’ time, maintaining below 5 years, whilst it enables a reliable ORC efficiency output and an optimal ORC power sizing, turning into a very interesting solution to boost waste heat recovery within industry.

In a nutshell, CIC energiGUNE contributes to make industrial waste heat recovery a feasible approach, not only to improve our environmental industrial legacy but also to go one step forward in industrial competitiveness. In CIC energiGUNE we join the best experts and knowledge on the field to make it real and provide the best solution for each particular case.

It is time to join for a more sustainable and green industrial revolution.