Heat recovery for high temperature reuse
State
Heat recovery is one of the most important actions that can be taken to increase energy efficiency in end-use, with a direct benefit on reducing CO2 emissions in the environment. In addition to recovery for direct use, in recent years technologies have been developed that can recover and reuse heat from industrial processes, especially when the amounts of energy involved and thermal levels are high, leading to a favourable energy and economic balance and the possibility of exiting the ETS constraints.
The best way to make thermal recovery effective and convenient is to identify the sources from which to draw the heat, accumulating it to make it available to cover the final needs when necessary.
Target
Following the entry into the market of new technologies increasingly performing, it is possible to integrate different equipment to achieve an optimization of results still not quantifiable, such as thermal storage and high temperature heat pumps. These machines have the same function as conventional heat pumps but allow to raise the temperature of a heat transfer fluid above 120 ºC, having a source at temperatures between 50 and 70 on the cold side and C. These characteristics allow the recovery and accumulation of heat at medium temperature and use it as a 'cold source' at the inlet to the heat pump, to raise the thermal level of a working fluid that will now have a wider use than usual. In this way it becomes possible to cover part of the needs of a process that requires temperatures between 120-150 C and the thermal recovery will no longer be used exclusively for low temperature users, tendentially related to the residential sector.
General description
The system consists of a PCM thermal battery combined with a high temperature heat pump, all of which is sized according to the temperature profile of the source and the need of end uses. When the thermal availability is higher than the demand, the excess energy is stored in the battery, which will be used at times when the recovery heat has characteristics lower than the operating specifications. This increases the working time of the machine maximizing thermal recovery.
The design criteria remain the same for all applications like the one proposed, to ensure the best result it is important to know both the profiles of thermal energy available and those of the needs; the ideal would be to have this information from direct measurements, but, alternatively, you can proceed with indirect processing.
This is the energy budget scheme used for the case study:
Data Analysis
The starting data that allowed the technical and economic evaluation of the intervention were the temperature of the condensate water before entering the final collection tank, for a typical day.
Below the graph with the trend over time.
Three zones are identified: until 8:20 the available temperature is higher than the input in the heat pump, from 8:20 until 17:00 the available temperature is lower than that required while after 17:00:We are once again in the position where we have greater energy availability.
The amount of energy available to charge the battery in 24 hours is higher than required in the period in which you have a lower temperature so you have the possibility to work continuously the system, even considering thermal losses that are not explicitly present, but can be covered by the energy surplus. Fully covering the needs maximizes thermal recovery, but requires a battery with a size of just over 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 the period when temperatures allow the machinery to work with an energy efficiency equal to the nominal, that is, when the temperature is above 70 ºC. To ensure maximum benefit throughout the day, a storage battery is expected to charge at times when there is surplus of thermal energy at temperature conditions above 70 ºC and then be reused as source of the heat pump, when the temperature drops below the working threshold. The same scheme can be replicated by adapting it to different working conditions, maintaining the objective of enhancing energy that would otherwise be wasted.
The thermal battery can be sized according to the amount of energy that you want to recover, taking into account that its installation in modules ensures flexibility over time and thus giving the possibility to subsequently expand the size of the accumulation.
Below the table containing the proposed plant characteristics:
Heat pump
Thermal battery
| Electrical power | 30 kWe |
| Thermal power | 120 kW |
| T in cold | 70°C |
| T out cold | 55°C |
| T in hot | 90°C |
| T out hot | 140°C |
| Storage size | 150 kWht |
| Thermal power | 150 kWt |
| T in (charge phase) | >75°C |
| T out (discharge phase) | 70°C |
The results expected from the thermal recovery and its use revalorized as a thermal level, are expressed on a daily and annual basis in the following table:
Annual balance (5gg x 48 wk)
Daily balance (3 shift)
| Thermal Energy recovered @70°C | 520 MWht |
| Thermal Energy valorized @150°C | 695 MWh |
| CO2 saved | 265.000 kg |
| Thermal Energy recovered @70°C | 1.850 kWht |
| Thermal Energy valorized @150°C | 2.450 kWht |
| HP total working hoursC | >20 h |
Technical and Economical evaluation
The economic assessment of the present case study was carried out taking into account the following elements, basing the calculations on the technical results expressed above and referring to average unit market costs:
- Reduced consumption of methane gas avoided
- Increase in electricity consumption from the heat pump grid
- Enhancement of white certificates for 5 years
- Investment cost of heat pump, thermal battery and installation
- Maintenance costs of the new plant
In the first approximation the relationship between the total costs of the investment and the result between rising and ending operating costs, lead to a simple return of the investment between 4 and 5 years.
Conclusions
Proceed with actions in which the thermal recovery, low enthalpy, of the plant energy flows that would otherwise be wasted, represent nowadays one of the most interesting solutions also as an option to reduce the mandatory quotas of the ETS Directive.
The use of thermal batteries allows to maximize the result due to thermal recovery especially in industrial processes where the variability of batch-type processes requires a greater flexibility of use of generation systems.