Heat recovery for high temperature reuse
Situation
Heat recovery represents one of the most important actions that can be implemented to increase energy efficiency in end-uses, with a direct benefit on the reduction of CO2 emissions into the environment. In addition to recovery for direct use, technologies have been developed in recent years capable of recovering and reusing heat from industrial processes, especially when the amounts of energy involved and thermal levels are high, leading to a favorable energy and economic balance and the possibility of exempting from ETS constraints.
The best way to make thermal recovery effective and convenient is the identification of sources from which to draw heat, storing it to make it available to cover final needs when necessary.
Objective
Following the market entry of increasingly high-performance new technologies, it is possible to integrate different equipment to achieve an optimization of results that is not yet quantifiable, such as thermal storage and high-temperature heat pumps. These machines have the same function as classic heat pumps but allow for raising the temperature of a heat transfer fluid above 120°C, having a source on the cold side with temperatures between 50 and 70°C. These characteristics allow for the recovery and accumulation of medium-temperature heat and its use as a 'cold source' at the heat pump inlet, to raise the thermal level of a working fluid that will now have a wider range of use than usual. In this way, it becomes possible to cover part of the requirements of a process demanding temperatures between 120-150°C, and thermal recovery will no longer be used exclusively for low-temperature users, typically linked to the residential sector.
General Description
The system consists of a PCM thermal battery combined with a high-temperature heat pump, all sized based on the source temperature profile and the end-use requirements. When thermal availability exceeds demand, the excess energy is stored in the battery, which will be used when the recovery heat characteristics are below operating specifications. In this way, the machine's operating time is increased, maximizing thermal recovery.
The design criteria remain the same for all applications similar to the one proposed; to ensure the best result, it is important to know both the available thermal energy profiles and the demand profiles. Ideally, this information should be obtained from direct measurements, but alternatively, one can proceed with indirect processing.
This is the energy balance scheme used for the case study:
Data Analysis
The starting data that allowed for the technical and economic evaluation of the intervention were the condensate water temperatures before entering the final collection tank, for a typical day.
Below is the graph showing the trend over time.
Three zones are identified: until 8:20 AM the available temperature is higher than the heat pump inlet temperature; from 8:20 AM to 5:00 PM the available temperature is lower than required; while after 5:00 PM the condition of higher energy availability returns.
The amount of energy available for battery charging over 24 hours is higher than that required during the period of lower temperature, thus allowing the system to operate continuously, even considering thermal losses which are not explicitly shown but can be covered by the energy surplus. Fully covering the demand maximizes thermal recovery but requires a battery size slightly exceeding 300 kWh.
Proposal and expected results
In the proposed case, a plant configuration has been designed where the heat pump directly uses waste heat during periods when temperatures allow the machinery to operate with nominal energy efficiency, i.e., when the temperature is above 70°C. To ensure maximum benefit throughout the day, a storage battery is provided that charges when there is a surplus of thermal energy at temperatures above 70°C, to then be reused as a source for the heat pump when the temperature falls below the operating threshold. The same scheme can be replicated by adapting it to different working conditions, maintaining the goal of valorizing energy that would otherwise be wasted.
The thermal battery can be sized based on the amount of energy to be recovered, taking into account that its modular installation guarantees flexibility over time, thus providing the possibility of subsequently expanding the storage size.
Below is the table containing the proposed plant characteristics:
Heat pump
| Electrical power | 30 kWe |
| Thermal power | 120 kW |
| Cold inlet T | 70°C |
| Cold outlet T | 55°C |
| Hot inlet T | 90°C |
| Hot outlet T | 140°C |
Thermal battery
| Storage capacity | 150 kWht |
| Thermal power | 150 kWt |
| Inlet T (charging) | >75°C |
| Outlet T (discharging) | 70°C |
The expected results from thermal recovery and its valorized use as a thermal level are expressed on a daily and annual basis in the following table:
Annual balance (5 days x 48 weeks)
| Recovered Thermal Energy @70°C | 520 MWht |
| Valorized Thermal Energy @150°C | 695 MWh |
| CO2 saved | 265,000 kg |
Daily battery balance (over 3 shifts)
| Recovered Thermal Energy @70°C | 1,850 kWht |
| Valorized Thermal Energy @150°C | 2,450 kWht |
| Total HP operating hours | >20 h |
Technical and Economic Evaluation
The economic evaluation of the case study under consideration was carried out taking into account the elements listed below, basing the calculations on the previously expressed technical results and referring to average market unit costs:
- Avoided natural gas consumption reduction
- Increase in electricity consumption taken from the grid for the heat pump
- Valorization of Energy Efficiency Certificates (White Certificates) for 5 years
- Investment cost of the heat pump, thermal battery, and installation
- Maintenance costs of the new plant
As a first approximation, the ratio between the total investment costs and the balance between new and discontinued operating costs leads to a simple return on investment (ROI) of between 4 and 5 years.
Conclusions
Proceeding with actions involving low-enthalpy thermal recovery of plant energy flows that would otherwise be wasted represents one of the most interesting solutions today, also as an option to reduce mandatory quotas under the ETS directive.
The use of thermal batteries allows for maximizing results from thermal recovery, especially in industrial process environments where the variability of batch processes requires greater flexibility in the use of generation systems.
