The desire to make a contribution in terms of environmental sustainability and the reduction of harmful emissions is spreading faster and faster, also thanks to the continuous development of new technologies. For many years, cogeneration has been one of the technologies with the greatest added value in terms of improving energy efficiency, because it combines the simultaneous production of electricity and heat. To date, machines with medium and large dimensions are more widespread, despite the fact that micro-cogeneration systems suitable not only for industrial and tertiary applications but also for the residential one are increasingly present on the market.
To make the most of the efficiency characteristics of the cogeneration machines, it is essential that the design of the entire equipment, especially towards thermal requirements, allows the system to work for the greatest number of hours per year, avoiding that heat is dissipated towards the environment.
To ensure greater flexibility in the relationship between the thermal production of the cogenerator and the user’s thermal consumption profile, the combination with phase change material (PCM) thermal batteries is envisaged.
Thanks to our devices, the heat is transferred to a specific storage medium, is stored and then returned when necessary; these three phases are not necessarily distinct from each other, they can in fact be simultaneous and repeated within the same cycle. An accumulation system is characterized with some fundamental parameters:
- efficiency, given by the ratio between the energy discharged and that stored
- the storage capacity, that is the quantity of energy that can be stored
- the power with which the storage is able to release the stored energy
- the length of time within which the energy can be stored inside the battery without significant energy losses
The combination of the thermal storage can be achieved on both large and smaller systems; in the first case, since it is a question of complex applications (industrial or advanced tertiary), it is necessary to proceed according to a storage model distribution to the sub-users with less constant withdrawal profiles, while for micro-cogeneration plants it is possible to directly install the thermal storage downstream of production heat.
For simplicity of calculation, this case study will refer to an application with a micro-cogenerator.
In general, to correctly design a cogeneration plant, it is essential to know the profiles (hourly) of both electrical and thermal consumption, becuase they are the most critical and binding factors in the final sizing of the machine. The higher the precision of the starting data, the more certain the result will be obtained, especially from the point of view of the economic return.
Figure 1. Percentage profile case study
Figure 2. Absolute profile case study
The use of a PCM battery allows you to act on the thermal profile in an active way, making the battery itself an active component to the overall needs of the user. An integrated system such as the one proposed can see in addition to the combination of cogeneration and storage also the insertion of a heat pump that would be powered by electricity supplied by the cogenerator itself. Thanks to the integration of the different devices: cogeneration + heat pump + thermal battery, it is possible to minimize the size of the generators by covering the basic thermal loads for the maximum number of hours possible.
Figure 3. Energy Contribution per device
Figure 4. Thermal Energy balance
The right sizing of the individual components is determined by the specific needs of the application by combining the user load profiles and the functional characteristics of the devices.
Figure 5. Plant Scheme
Once the design and construction part has been completed, the thermal battery by its nature is characterized by a simple installation and an equally simple management with a very low maintenance, being a device consisting only of static components with a very long life (> 15 years). At the end of its life, the bio phase change material does not involve particular criticalities for its disposal.
Figure 6. Thermal battery photo
Technical and economical evaluation summary
Taking into account that each application must be evaluated both from a technical and economic point of view, the result of an analysis performed on a customer in the tertiary sector, characterized by a basic thermal use that extends beyond the thermal season (October / April) with non-constant daily consumption profiles.
In the following table, the main information:
MICRO CHP: 10 Kw
HEAT PUMP: 4 kW
PCM TERMAL BATTERY: 60 kWh
TOTAL INVESTMENT COST: € 60.000
GRANT (italian tax deduction) : 65%
WORKING HOURS: 6.500
SPBT: 5,1 years