A group of researchers from Spain's University of the Basque Country (UPV/EHU) has designed a building-integrated photovoltaic forced ventilated facade (PV-FVF) to be used as support for heating and a domestic hot water (DHW) building system based on air source heat pumps (ASHPs).
“Our system design is cost-competitive compared to a ventilated façade made of ceramic tiles,” the research project's corresponding author, Carlos García-Gáfaro, told pv magazine. “The additional costs for the photovoltaic system and its connection to the heat pump are amortized by the increased efficiency of the building energy system,” he also explained, noting that the overall system costs may be up to 20% higher compared to a system without the PV installation.
The system was simulated as being deployed at the Quality Control Laboratory of the Basque Government Building (LCCE) in the city of Vitoria-Gasteiz in northern Spain. The Paslink test cell and methodology were used to assess the thermal response of the building element in exterior conditions.
The PV-FVF is an architectural solution proposal consisting of a base wall with an insulating layer that is covered with a ventilated façade with a forced airflow chamber and PV-glass elements in the exterior layer. The ventilated façade had six 0.65 cm-thick 5%-efficient opaque photovoltaic glass layers based on amorphous silicon inserted between glass plates that were integrated with the building's structural load-bearing parts with an air gap of 10 cm. The connection between the PV-FVF and the heat pump is possible with normal metal ducts from the heating, ventilation, and air conditioning (HVAC) industry. The provider of the PV glass technology is Spanish specialist Onyx Solar.
In the proposed system configuration, the heat pump is expected to cover building heating demand at all times, regardless of the performance of the PV-FVF unit. “When the PV-FVF airflow extraction is running at a high velocity, the heat pump will function with greater efficiency. On cloudy days or at night, the PV-FVF functions on a low flow basis of 10 l/s. In such periods, the heat pump uses exterior air mixed with the said small flow coming from the PV-FVF,” the scientists explained, adding that the entire building is used as a thermal storage system by the setpoint temperature schedule, using one strategy named Tadaptive. “Its objective is to transfer the maximum thermal energy possible to the building when the façade is obtaining a higher solar thermal gain,” García-Gáfaro stated.
When airflow is induced through the wall's ventilated chamber, the PV-FVF element is able to act as a heating tunnel. The outer surface of the insulating layer is warmer at nighttime and the inner side of the PV layer is warmer during daytime periods. This, according to the scientists, has a positive effect on the convective processes of the airflow, as the convective heat transfer coefficient of the warmer surface is higher. The airflow was found to quickly reach the developed flow condition, both in temperature and velocity.
The system was simulated in a 54-dwelling residential building equipped with a heat pump for both space and water heating located in Madrid. “The results obtained indicate that the integration of PV-FVF and heat pump, combined with an underfloor heating system, reduces heating energy consumption by at least 19.9% and, coupled with the photovoltaic generation of the outer layer, reduces the annual total energy consumption, including domestic hot water, by at least 20.7%.”
The proposed system configuration is introduced in the study A photovoltaic forced ventilated façade (PV-FVF) as heat source for a heat pump: Assessing its energetical profit in nZEB buildings, which was recently published in Energy and Buildings. “Our model indicates that the extra costs represented by the PV system may be repaid in only four years,” García-Gáfaro concluded. “Further research is planned for this system. First, the assembly and monitoring of a prototype in an office building would be carried out. Later, the use of the airflow after passing through the heat pump as support of the heat recovery ventilation system will be evaluated.”
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