Jean Castaing-Lasvignottes

Solar Domestic Hot Water (DHW) systems are crucial for reducing the environmental impact of the residential building sector, particularly in regions with ambitious renewable energy goals, such as La Réunion. However, traditional collective solar thermal installations face significant implementation barriers in building retrofits due to complex piping networks, high maintenance costs, and the risk of water leakage. This paper presents a novel alternative: a rooftop photovoltaic-powered domestic hot water (DHW-PV) system specifically designed for multi-unit housing. A first DHW-PV system features a common 41.58 kWp PV array directly connected to 35 individual electric water heaters via a direct current wire network. Energy distribution and Maximum Power Point Tracking are managed by an innovative, low-cost Energy Management System utilizing simple contactors and a dynamic equity index based on each dwelling's hot water demand profile. To assess its performance, a real-world multi-unit residential building equipped with the DHW-PV was fully instrumented and monitored. The analysis of typical summer and winter days shows the strong influence of tank thermostat states on solar conversion efficiency. It also highlights that while high solar availability in summer leads to fully cover the user needs and low PV Performance Ratios (PR), winter cloud cover could result in DHW deficits. Over an entire year of monitoring, the system covered 91\% of DHW tenants' needs, matching the original design target with an annual PR of 62\% suggesting an oversized configuration. These findings establish the techno viability and scalability of DHW-PV as a robust solution for renovating DHW systems in collective housing.

Facing the demand in electricity for houses or buildings provided by mean of photovoltaic panels is relatively tricky, especially because of the stochastic character of the solar radiation. There exists some solutions in terms of storage and among them, the one consisting in converting electricity in high-pressure compressed air seems promising. This option is under investigation in a building of teaching classes at Reunion University (a French island situated in the Indian Ocean, at the east of Madagascar. The aim in terms of consumption is 25 kWhPE.m-2.year-1 leading, if possible to a NetZEB (Zero Energy Building). So, it has been envisaged to produce the electricity by mean of PV panels, to consume the major and to store the extra production in order to be used at night and during lower shinning periods. The solution that has been investigated consists in compressing air in high pressure storage tanks and to produce electricity later thanks to a turbine. A dynamic numerical model has been built, considering a specific given load profile and local climatic data in order to estimate the production. The whole system depends mainly on the PV panel area, the storage tank volume and the pressure range. The objective is to reduce the amount of energy provided by the external network to the minimum leading to the maximum autonomy of this system. First results have been obtained and are explained in a last part, in terms of pressure and power evolution.