From Problem to Opportunity
As discussed in the previous article, carbon capture systems—though essential for decarbonization—lead to a reduction of a plant’s net power output. This occurs because part of the steam extracted from the turbine is redirected to provide thermal energy for solvent regeneration in the CO₂ capture unit. This reduction is known as the energy penalty.
In sound engineering practice, we should look at this process more broadly. The energy consumed in CO₂ capture does not disappear; it is degraded. A significant portion of it is released as low-grade waste heat, with low exergy and limited potential to do mechanical work. Nevertheless, it remains a valuable resource.
If the temperature of this waste heat is higher than the plant’s lowest energy level (e.g., the enthalpy of expanded steam leaving the turbine), good engineering practice dictates that it should be utilized. The goal of modern energy efficiency is to convert waste heat into a useful product rather than letting it dissipate.
Waste Heat from CO₂ Capture – A Resource Waiting to Be Used
Post-combustion CO₂ capture, particularly in amine-based systems, produces large quantities of low-temperature heat (typically 50–70°C). This heat comes from:
- cooling flue gases in the DCC (Direct Contact Cooler),
- condensation of water vapor and process condensate,
- cooling of auxiliary circuits.
In most installations, this energy is simply rejected to the cooling water system or discharged to the atmosphere through air coolers, representing tens of megawatts of lost thermal power. While this heat has low exergy, it can still be useful—provided it is upgraded to a higher temperature level.
Heat Pumps – Raising the Value of Energy
In Combined Heat and Power (CHP) plants, waste heat recovery opportunities are particularly attractive because of the presence of an existing district heating network. Such low-grade heat cannot be used for power generation, but it can be upgraded to the return or even supply level of the district heating system.
This puts CHP plants in an advantageous position: they have access to both turbine extraction steam and fresh steam that can supply heat pumps. Depending on configuration, these heat pumps can be driven by steam or electricity.
Absorption heat pumps (AHP) use heat as their driving force instead of electricity. They typically employ a LiBr–H₂O working pair and are powered by medium-pressure steam (around 8–12 bar). Their Coefficient of Performance (COP) is approximately 1.7, meaning that for each unit of input heat, nearly twice as much useful heat can be recovered.
Electric heat pumps (EHP) operate differently—they require electrical energy as their power source. In CHP systems, this electricity is generated with limited efficiency (typically 30–40%). Despite that, with a COP of around 3.0, EHPs can raise waste heat temperatures up to 115°C, allowing supply to higher-temperature district heating grids.
System-Level Effects – Less Waste, More Value
For CHP plants, the impact of heat recovery integration must be considered at the system level. The potential benefits depend on load profiles, seasonal variability, the presence of heat storage, and market energy prices. In general, waste heat upgrading with heat pumps enables:
- reduced consumption of turbine extraction steam (and, in some configurations, reduced use of fresh steam feeding network heat exchangers),
- increased share of low-temperature heat in the district heating balance (especially for base load coverage),
- flattening of thermal peaks with the help of heat storage, when available, and
- reduction of the equivalent electrical effort associated with steam extraction.
The achievable results depend on factors such as DCC temperatures and flow rates, district heating return/supply temperatures, availability of driving steam (for AHPs) or electrical power (for EHPs), and the control strategy. Each district heating system has its own characteristics, so a dedicated concept-level study is essential (concept → integration options → control and capacity sizing).
Mitigating the Energy Penalty – Realistic Benefits
Waste heat recovery does not eliminate the energy penalty—it helps mitigate it. By integrating heat pumps and recovery loops, it is possible to partially compensate for the thermal and electrical losses that CO₂ capture introduces. The overall energy balance of the plant improves, while the residual loss remains technically manageable.
Case Study: 70 MW CHP Integration – Results from the WEProS Feasibility Study
The feasibility study carried out by WEProS for a 70 MW thermal CHP plant, supplying up to 25 MW to a district heating network, demonstrated that the overall energy penalty of CO₂ capture (≈ 25 %) can be substantially reduced through integrated waste‑heat recovery.
When applying absorption heat pumps (AHP) powered by 12 bar steam, combined with district‑heating return heat upgrading, the penalty was reduced to about 6–7 %, representing a 70–75 % reduction of losses. For electric heat pumps (EHP) with COP ≈ 3.0, the reduction reached 35–40 %, lowering the penalty to roughly 15 %.
These results already include the additional electricity required for EHPs and the extra steam consumption for AHPs. The corresponding payback period was approximately one year for the absorption‑based configuration and three to four years for the electric option.
It is important to note that these figures depend strongly on plant‑specific conditions — heat demand profiles, steam parameters, and integration strategy. Under favorable configurations, as in this case study, such results are achievable.
Toward Sustainable Energy Integration
The key lesson from this work is that waste-heat recovery cannot eliminate the energy penalty, but it can significantly moderate it — often by more than half. Every plant’s configuration is unique, and results depend on how creatively energy flows are integrated.
If you are curious about how much your facility could save or how far its energy penalty could be reduced, WEProS can support you in assessing the technical and economic potential through a dedicated feasibility study or by assisting your project team in the conceptual and integration phases.