The aim of this paper is the evaluation of the profitability of micro-CHP systems for residential application. An integrated CHP system composed of a prime mover, an Electric Energy Storage system, a thermal storage system and an auxiliary boiler has been considered. The study has been carried out taking into account a particular electrochemical storage system which requires also thermal energy, during its operation, for a better exploitation of the residual heat discharged by the prime mover. The prime mover could be a conventional Internal Combustion Engine or also an innovative system, such as fuel cell or organic Rankine cycle.
An investigation of this integrated CHP system has been carried out, by means of an in-house developed calculation code, performing a thermo-economic analysis. This paper provides useful results, in order to define the optimum sizing of components of the integrated CHP system under investigation; the developed code allows also to evaluate the profitability and the primary energy saving with respect to the separate production of electricity and heat.
Among the approaches to achieve the targets of primary energy saving and greenhouse gas reduction [1], combined heat and power (CHP) generation is a feasible strategy, recognized and supported by the European Union [2]. The potential and actual convenience of CHP systems strongly depends on the specific techno-economic scenario in which the CHP system operates. Several studies can be found in literature concerning the assessment of CHP advantages–disadvantages from the energy/environmental/economic points of view and in various applications [3], [4], [5], [6], [7], [8], [9], [10], [11] and [12]. The analyses carried out here refer to the case of a residential building application and to a typical Italian economic tariff scenario, but the methodology could be extended to other market scenarios.
The applications of CHP technologies to the residential sector is still limited and, as pointed out in a previous study of the authors [13], the correct sizing of components is a critical aspect which should be properly defined, in order to maximize the system profitability and energy saving. In particular, among the key factors affecting the CHP profitability, a proper management of the energy flows between the prime mover and the storage subsystems should be considered. In the previous study [13], an investigation had been carried out by considering the external electric net as the only available system to store in the surplus of electric energy and to withdraw from the additional requested power. In this study, the presence of an additional electrochemical battery working as Electric Energy Storage system (EES) is considered. The EES helps to decouple the production and utilization of electricity and it is useful in the energy systems in which at least one of the following conditions occurs:
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The electricity production prediction is limited by external non-programmable conditions (e.g., availability of renewable source energy systems, such as wind or solar energy).
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The external electric network is not available, or with limited capacity.
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The power generator operates at fixed point and at full power, to respect the maximum efficiency conditions, or for other reasons linked with the generator characteristics (e.g., in high temperature fuel cells, the large internal thermal inertia do not allow the system to comply with rapid load changes).
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The electric power requested by the utilities undergoes rapid and strong changes due to its stochastic components and the power generator can hardly follow the fluctuating request.
Moreover, the electric energy request is usually not directly correlated with the thermal energy request. Thus, also a Thermal Energy Storage system (TES) can be used in a CHP system, in order to store the available thermal energy not absorbed by the utilities during the operating periods in which thermal surplus occurs, instead of dissipating it, with the aim to increase the primary energy saving. In the preliminary study [13] the TES minimum size had been linked to the CHP prime mover power size and the thermal user request. Results of the previous study concerning the TES sizing will be used also within this paper.
More in general, the sizing of both the EES and the TES is affected by the user energy request versus time profiles, by the size of the prime mover and by the electricity tariffs, as it is shown in this paper. In the first section, the considered CHP system layout is described, taking into account the available, developed or under development, technologies for prime movers and for electricity storage devices; in the following section, the methodology of the carried out analysis is introduced, describing the user scenario and the used CHP generators and electrochemical batteries taken into consideration as EES. Finally, the obtained results are shown and a critical analysis is carried out.
The integration of a CHP prime mover provided with a TES and with an EES based on the ZEBRA technology has been investigated in this paper for residential application. The CHP system comprises also an auxiliary boiler and interconnection with the external electric net.
The energy benefits and the economic profitability of different micro-CHP prime movers have been linked to different sizes of the EES and taking into account a typical residential building electric and thermal energy demand, in order to identify general considerations on the proper sizing of the CHP prime mover and the EES.
The obtained results shows that CHP units, with appropriately sized EES and TES, can cover both the overall electric and thermal energy demand and can achieve a significant saving of primary energy in comparison with the reference case of non-CHP conditions.
The carried out parametric economic analysis highlights that key factors for the profitability of the CHP systems are the proper sizing of both the CHP prime mover and EES. Depending on the selected prime mover, the adoption of a properly sized ZEBRA battery can be feasible at the current economic scenario in Italy, if specific costs of the EES remain below few hundreds of Euros per unit of kW h.
