Stefania Perrella

In the EU, the building sector is responsible for 40% of the global energy consumption for final uses and 36% of the carbon dioxide (CO₂) emissions. Heat pumps allow for the replacement of conventional systems based on fossil fuels with the perspective of combining PV and solar thermal collectors. In order to rationalize the use of the solar source, this paper examined the self-consumption electricity share, the CO₂ equivalent emissions, and the domestic hot water demand covered by renewable sources which were determined in two opposite climatic conditions. These involved both electric and thermal storage systems and considered two different control strategies. The first is commonly used for the management of air-conditioning systems, the second was specifically conceived to maximize the exploitation of the solar source. Results showed that the latter significantly reduced grid dependence in both locations, determining the direct satisfaction of 76% of the thermal and electric loads through the PV self-consumption, determined by 18 kWp of installed PV and a battery capacity of 24 kWh. In terms of equivalent CO₂ emissions, when the two control strategies were compared, a remarkable reduction in emissions was registered for the latter, with percentages ranging between 8% and 36% as a function of PV surface and battery capacity. The analysis of domestic hot water supplies revealed disparities between the two localities: the colder first, relied more on heat pumps for water heating, while the warmer second, benefitted from the large availability of solar radiation.

Heat pumps in buildings allow for the limiting of CO₂ emissions by exploiting directly the renewable energy available in the external environment (aerothermal, hydrothermal and geothermal sources). Moreover, other renewable technologies such as active solar systems can be integrated easily into use with them. This combination not only increases the share of primary energy provided by renewable sources for heating/cooling but also improves the heat pump performance indices. Nevertheless, in cold climates, air–water heat pumps should be equally penalized due to the unfavorable outdoor air temperature. Conversely, a water–water heat pump, connected with a solar tank and thermal solar collectors, overcomes this issue. Indeed, the higher temperature attainable in the cold source allows for reaching greater COPs, and when the solar tank temperature level is enough, emitters can be directly supplied, avoiding the absorption of electric energy. In this paper, this plant configuration, in which a further tank after the heat pump was considered to manage the produced thermal energy, is investigated. Proper control strategies have been developed to increase the renewable share. Regarding a reference residential building located in Milan, for which the water–water heat pump was sized properly, a parametric study, carried out in TRNSYS by varying solar tank volume and collecting surface, has allowed for the identification of the optimal system configuration. A renewable share, ranging between 54% and 61% as a function of the collecting surface and the storage volume, was detected, as was an average seasonal coefficient of performance (SCOP) over 4. Regarding two common heating plant configurations using an assisted PV air-to-water heat pump and a gas boiler, the optimal solution allows for the limiting of CO₂ emissions by 33% and 53%, respectively.

The ecological transition at the centre of the United Nations 2030 Agenda and the relevant EU policies are increasingly becoming an emerging issue in the political choices of most countries. It is an important challenge to ensure sustainable development and overcome the issue of energy supply. Italy produces 35% of its electricity consumption, a too low percentage that obligates the nation to purchase abroad to cover the overall needs. Energy communities can represent an interesting and viable option for businesses and citizens struggling with the abrupt rising of energy prices. In community energy systems, the energy demand of a group of households or public services is met by electricity collectively generated through renewable sources and this feature is particularly suggested in small towns to promote social benefits and environmental advantages. In this work, possible scenarios of an implementable energy community were investigated for the small mountain municipality of Soveria Mannelli, located in Southern Italy. A building stock made of four public edifices was used as a reference case for which heating needs were determined by dynamic simulations based on the EN ISO 52016-1 procedure. Other simulations carried out in the TRNSYS environment allowed for implementing different schemes of the energy community considering diverse building interaction modes, in which photovoltaic generators and electric batteries cooperate to supply heat pump systems to assure the maximum share of self-consumed electric energy. Indeed, this paper is targeted at the identification of the best solution in terms of technical and economic performance. Despite an evident study limitation is represented by the exclusive use of PV and electric storage systems, the results demonstrate a potential CO₂ emission reduction of over 80%. The more profitable solution for the Municipality was identified with an NPV of 11 k€ in 20 years with appreciable payback.

The Trombe wall is a passive system used in buildings that indirectly transfers thermal energy to the adjacent environment by radiation and convection, and directly by the thermo-circulation that arises in the air cavity delimited between a transparent and an absorbing surface. Nevertheless, the latter is painted black to increase the energy gains, but this produces a negative visual impact and promotes the overheating risk in summer. To mitigate these aspects, a hybrid Trombe wall equipped with PV panels can be employed. The PV installation results in a more pleasing wall appearance and the overheating risk reduces because part of the absorbed solar radiation is transformed into electricity. To determine the actual performance of a such system, transient simulation tools are required to consider properly the wall thermal storage features, variation of the optical properties, air thermo-circulation, and PV power production. Alternatively, regarding the traditional Trombe wall, the literature provides a simplified empirical method based on the dimensionless parameter solar load ratio (SLR) that allows for preliminary evaluations and design. In this paper, the SLR method was calibrated to determine the monthly auxiliary energy to be supplied in buildings equipped with PV-Trombe walls in heating applications. The SLR method was tuned by a multiple linear regression by data provided by TRNSYS simulation that allowed to obtain the energy performances in actual conditions of PV-Trombe walls installed on the same building but located in different localities. The comparison between the TRNSYS results and the calibrated SLR method determined average errors ranging between 0.7% and 1.4%, demonstrating the validity of the proposed methodology.

Low-energy buildings are generally equipped with generation systems driven by renewable sources. Regarding heating and DHW production, two choices appear appropriate: PV assisted heat pumps and biomass boilers. In this paper, by means of TRNSYS dynamic simulations, the non-renewable primary energy was determined for two buildings located in different climatic contexts by varying the PV size to consider the actual self-consumed electricity of commercial devices. Results showed that in cold climates biomass boilers are more suggested, especially in unfavourable climatic zones, whereas the COP of air-water heat pumps is strongly penalized by the outdoor temperatures and in many cases the self-consumed PV electricity does not limit the grid intervention adequately. However, in building with limited thermal energy demands and in favorable climates, suitable PV sizes make heat pumps more performant than biomass boilers. The same calculations were conducted with the quasi-steady approach, in accordance with the Italian building energy certification procedure, observing a favorable scenario in a heating plant equipped with a PV assisted heat pump because it assumes the renewable electricity entirely absorbed, while the accounting of the actual self-consumed share produces a greater demand of non-renewable energy.