Heat recovery is one of the most important actions that can be carried out 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 to be capable of recovering and reusing heat from industrial processes, especially when the quantities of energy involved and thermal levels are high, leading to a favorable energetic and economic balance and the possibility of reducing/exiting the ETS constraints.
The best way to make heat recovery effective and convenient is by identifying the sources from which to draw heat, accumulating it to make it available to cover final needs when necessary.
Following the entry on the market of new and increasingly performing technologies, it is possible to integrate different equipment to obtain previously unpredictable results, such as thermal storage and high-temperature heat pumps. These machines have the same function as classic heat pumps but allow the temperature of a heat transfer fluid to be raised above 120 °C, having a source at temperatures between 50 and 70 °C on the cold side. These characteristics allow to recover and accumulate heat at medium temperature and use it as a ‘cold source’ at the inlet of the heat pump, to raise the thermal level of a working fluid which will now have a wider use than usual. In this way it becomes possible to cover part of the requirements of a process that requires temperatures between 120-150 °C and heat recovery will no longer be used exclusively for low-temperature users, which tend to be linked to the residential sector.
The system consists of a PCM thermal battery combined with a high temperature heat pump, all dimensioned according to the temperature profile of the source and the needs of the end uses. When the heat availability is higher than the request, the excess energy is stored in the battery, which will be used when the recovery heat has characteristics lower than the operating specifications. In this way the working time of the machine is increased by maximizing the 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 those of the requirements; the ideal would be to have this information from direct measurements, alternatively you can proceed with indirect processing.
This is the diagram of the energy balance used for the case study:
The starting data that made it possible to perform 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.
The next graph shows a 24 h typical temperature profile of waste condensate.
Three zones are identified: up to 8:20 the available temperature is higher than that entering the heat pump, from 8:20 to 17:00 the available temperature is lower than the required one, while after 17:00 it is again in the condition in which there is greater energy availability.
The amount of energy available for charging the battery in 24 hours is higher than that required in the period in which there is a lower temperature, therefore it is possible to make the system work continuously, also considering the thermal losses that are not explicitly present but they can be covered by the energy surplus. Fully covering the requirement maximizes heat recovery but requires a battery with a size of just over 300 kWh.
Proposal and expected results
In the proposed case, a system configuration has been devised where the heat pump directly uses the waste heat in the period in which the temperatures allow the machinery to work with an energy efficiency equal to the nominal, i.e. when the temperature is higher than 70 °C. To ensure maximum benefit throughout the day, a storage battery is provided which charges when there is surplus of thermal energy at temperatures above 70 °C and then reused as a 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, while maintaining the goal of enhancing energy that otherwise would be wasted.
The thermal battery can be sized according to the amount of energy to be recovered, taking into account that its installation in modules guarantees flexibility over time and thus giving the possibility of subsequently expanding the size of the storage.
Below is the table containing the proposed system characteristics:
|Electric 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|
|Energy Storage Size||150 kWht|
|Thermal Power||150 kWt|
|T in (charging)||>75°C|
|T out (discharging)||70°C|
The expected results from heat recovery and its use revalued as a heat level are expressed on a daily and annual basis in the following table:
Annual Balance (5 days x 48 wk)
|Thermal Energy Recovered @70°C||520 MWht|
|Thermal Energy Supplied @150°C||695 MWh|
|Saved CO2||265.000 kg|
Daily Balance (over 3 shift)
|Thermal Energy Recovered @70°C||1.850 kWht|
|Thermal Energy Supplied @150°C||2.450 kWht|
|Heat Pump Total Working Hour||> 20 h|
The economic evaluation of the case study studied was carried out taking into account the elements listed below, basing the calculations on the technical results previously expressed and referring to average market unit costs:
- Reduction of methane gas consumption
- Increase in the consumption of electricity taken from the network for the heat pump
- Enhancement of white certificates (italian incentives) for 5 years
- Investment cost of the heat pump, of the thermal coil and of installation
- Maintenance costs of the new plant
As a first approximation, the ratio between the total investment costs and the result between rising and discontinuing operating costs, lead to a simple return on investment of between 4 and 5 years.
Proceeding with actions in which the thermal recovery, at low enthalpy, of the energy flows of the plant that otherwise would be wasted, is nowadays one of the most interesting solutions in term of energy efficiency also as an option to reduce the mandatory quotas of the ETS directive .
The use of thermal batteries allows you to maximize the result due to heat recovery, especially in industrial processes where the variability of batch-type processes requires greater flexibility in the use of generation systems.