Numerical investigation of the castle of Zena energy needs and a feasibility study for the implementation of electric and gas driven heat pump

The Italian buildings stock includes a large amount of historical buildings. The reduction of national building energy consumption necessarily implies their refurbishment as well as an effective use of appropriate HVAC technologies today available. In this work a numerical investigation of the historical building Castle of Zena (XXIII century) and a feasibility study for the retrofit of its HVAC plant is presented. The work followed three main steps: the definition of the numerical model and the simulation of the building energy performance using the software platform TRNSYS 16 (Transient System Simulation Tool); selection, design and simulation of three appropriate HVAC options; discussion of results from the energy, economic and environmental point of view. Two type of numerical analyses on the building envelope have been performed: a first one in free floating conditions (no temperature control), and a second one with an ideal temperature control. The free floating simulation has been used for a first evaluation of the building performance. The adaptive comfort approach (Adaptive Comfort Algorithm) has been implemented to compare internal temperature with an acceptability range. Results have shown that during almost the 50% of the summer season internal temperature is above the upper limit of the comfort range, therefore a cooling plant is needed. On the other end, the annual energy needs for space heating is around 164 kWh/m2/year, while the space cooling is around 5 kWh/m2/year. Results coming from this first analysis, together with data obtained with the simulation with controlled internal conditions, have been used to design the HVAC systems. Due to the historical constraints and the installation effort needed it has been decided to exclude the HVAC technologies belonging to the category of all-air systems. Water systems have been selected. In particular, among them two pipe fan coil units were chosen as emissions systems, characterized by easy integration despite the good emission efficiency. Given the environmental and landscape constraints, wind, photovoltaic and solar thermal technologies have been excluded. Therefore a heat pump system was chosen as generation system using renewable energy. In the present study, two different type of heat pump system (HP) are analyzed. The first one is an electric heat pump (EHP) based on a compression cycle electrically powered and the second one is an air source gas driven absorption heat pump (GAHP-AS). Moreover two types of EHP systems have been analyzed. The first one is an air-water heat pump (EHP-AS) characterized by a low initial investment cost, while the second one is a water source heat pump (EHP-WS) linked to the existing well of the Castle. Neglecting, the control, emission and distribution sub-system energy needs, the system with the lowest primary energy consumption is the EHP-WS (103 kWh/m2/year), followed by the GAHP-AS (150 kWh/m2/year) and the EHP-AS (180 kWh/m2/year). From the economic point of view the EHP-WS have the shortest pay-back time, 7 years, also thanks to the use of the existing well, against the 14 years of the GAHP-AS. However, excluding the water source, only the gas absorption heat pump is economically feasible. Finally we could obtain a significant reduction of greenhouse gas emission (CO2) replacing the EHP-AS with the GAHP-AS (-26%) or the EHP-WS (-38%).