Environmental concerns worldwide (climate change, global warming, etc.) are pushing to reduce the consumption of fossil fuels. The building sector is responsible for a third of greenhouse gas emissions (40% in France). In tropical countries, the main share of energy consumption in buildings is due to air conditioning systems. Indeed, in a resort of high standing, 60% of energy consumption is due to air conditioning. In the Indonesian context, which welcomes growing real estate projects on more or less isolated islands, it becomes important to put in place passive or autonomous buildings and the corresponding energy solutions. The energy efficiency of buildings is based on two pillars: an efficient building's design and on the effectiveness of the air conditioning system to achieve energy independency in a tropical environment. Considering the decreasing cost of PV cells, the solution to reduce the energy consumption of air conditioning proposed in this article covers a vapour-compression refrigeration system electrically powered by solar cells. To avoid the use of electric batteries, not sustainable in terms of carbon footprint (construction and recycling of batteries) and to overcome the problem of intermittency of solar energy, the choice fell on a variable speed compressor and a storage in a mixture of fatty acids (derived from coconut oil) as phase change material embedded in expanded graphite. The work also focuses on the energy performance of the storage system. This study describes the context and the air conditioning system chosen as a solution for a sustainable resort application in a tropical region. The design and characterization of the coupled PCM and compressed expanded graphite in a latent heat thermal energy storage is also detailed. It uses a TRNSYS simulation for the assessment of the cooling demand. Calculations for a prototype of 25 m 2 apartment showed that with a chiller of 8000 W and a surface of 14 m 2 of photovoltaic panels, it is possible to cool a hotel bedroom with solar energy. The consortium members work jointly at designing and optimizing the system: Indonesian members are focused on the PCM storage and French members are more dedicated to the hygrothermal behaviour of the hotel bedrooms.

A fast-paced energy transition needs a higher penetration of renewables, of heating and cooling in the worldwide energy mix. With three novelties 1-of using shallow high-pressure LRC (Lined Rock Cavern) excavated close to storage needs, 2-of using a slow-moving CO2 piston applying steady pressure on the hydro part of UPHES (Underground Pumped Hydro Energy Storage) and 3-of relying on inexpensive thermal stores for long-duration storage, CO2 UPHES coupled with PTES (Pumped Thermal Electricity Storage) could become, at expected Capex cost of only 20 USD/kWh electrical, a game-changer by allowing the complete integration of intermittent renewable sources. Moreover, even though this early conceptual work requires validation by simulation and experimentation, CO2 UPHES as well as UPHES-PTES hybrid storage could also allow a low-cost and low-emission integration of intermittent renewables with future district heating and cooling networks.

The urgent energy transition needs a better penetration of renewable energy in the world’s energy mix. The intermittency of renewables requires the use of longer-term storage. The present system uses water displacement, in a lined rock cavern or in an aerial pressurised vessel, as the virtual piston of compressor and expander functions in a carbon dioxide heat pump cycle (HPC) and in an organic transcritical cycle (OTC). Within an impermeable membrane, carbon dioxide is compressed and expanded by filling and emptying pumped-hydro water. Carbon dioxide exchanges heat with two atmospheric thermal storage pits. The hot fluid and ice pits are charged by the HPC when renewable energy becomes available and discharged by the OTC when electricity is needed. A numerical model was built to replicate the system’s losses and to calculate its round-trip efficiency (RTE). A subsequent parametric study highlights key parameters for sizing and optimisation. With an expected RTE of around 70%, this CO2 PHES (pumped-hydro electricity storage) coupled with PTES (pumped thermal energy storage) could become a game-changer by allowing the efficient storage of intermittent renewable energy and by integrating with district heating and cooling networks, as required by cities and industry in the future.

No abstract

No abstract