Can waste heat be converted into electricity?
Industrial processes often yield excess heat which cannot be used in the process any further. Technologies exist which can convert such “waste heat” into useful energy, such as heat pumps or ORC-units. A heat pump allows, at a low energy cost, to upgrade low temperature heat into high temperature heat. An organic Rankine cycle (ORC) transforms low temperature heat into mechanical work, which can be used to generate electricity.
(check out https://en.wikipedia.org/wiki/Organic_Rankine_cycle)
But can you do it profitably?
While waste heat conversion makes sense from an energy point of view, a key question is if it will fly economically? Take the case of an ORC. The excess heat is considered as free of charge and the electricity produced yields additional revenue, however the equipment (turbine, heat exchanger, electricals, civils) requires a significant initial investment and recurring O&M (operations and maintenance) costs. Is the improved energy efficiency worth the trouble?
Project feasibility lines can help
The total investment cost is very project dependent for an ORC and often not readily known. However, the number of operating hours per year (hours that excess heat is available), as well as the value of a MWh of electricity, can be estimated easily. These are the main drivers on the revenue side. In a project feasibility plot, we can plot the operating hours and electricity cost needed to reach a specific IRR (here 10%), assuming a fixed investment cost (capex, expressed in €/kWe). The plotted project feasibility lines helps to rephrase the feasibility question: what is the maximum investment (per kW of electrical capacity of the ORC unit) that the project can bear?
Assume heat is available for about 6.000 hours per year (e.g. because the process is running at full capacity 24/7, but only during weekdays). Also assume that the average revenue from electricity is 140€/MWh (or 80€/MWh with 60€/MWh government support of some sort). From the chart, it can readily be seen that the maximum project cost is limited to 4000€/kW installed electrical capacity. Here you have a benchmark which is easier to assess.
ORC or not?
Knowing that an ORC is a kind of “steam turbine” (Rankine cycle) with a relatively low thermodynamic efficiency by nature due to the low temperatures involved, beating the 3000€/kW line is a tough cookie. As much as we at Entras would love to see more waste heat recovered in industry, an ORC is not the silver bullet.
But if you think you might have a case, let us know.