The use of renewable energy sources to produce thermal energy is one of the key actions of the energy transition for reducing carbon emissions to the atmosphere. On-site generation of thermal energy from renewables can help local municipalities, cities and communities harness substantial environmental, economic and social benefits. The main technologies used for renewables thermal energy generation which can be adopted in cities are:
Solar thermal – It represents the most common solution to produce thermal energy . Solar thermal collectors are devices that convert solar energy into thermal energy available in a working fluid. Solar thermal collectors can be work for both small-scale and large-scale applications, depending on the technology. Solar thermal collectors can also be integrated into the building envelope (BIST). The easiest way to classify solar collectors is to divide them into concentrated and non-concentrated solar collectors. Under the first category, solar thermal collectors can be distinguished according to the heat transfer fluid used, which is typically air, water, or a refrigerant. The most common uses include:
Solar water heating systems: used for residential and non-residential applications to cover space heating or domestic hot water (DHW) demands.
Solar district heating: large scale solar collectors can be employed to provide heating to district heating networks.
Solar thermal cooling: solar thermal collectors can be coupled with absorption chillers to provide cooling energy for residential use. Due to the high temperature to drive chillers, evacuated tubes or concentrators are usually employed.
Solar heat for industrial processes: solar thermal collectors can be used to supply heat for different industrial processes including textile, food, and agricultural applications.
Fresnel solar thermal system
Geothermal : The thermal energy contained in the deep earth crust can be used both for heating/cooling applications. The most common use of geothermal energy for thermal energy production are geothermal heat pumps (GSHP), which extract the energy from the ground to support the evaporation/condensation of the refrigerant achieving a higher efficiency compared to air source heat pumps. In a conventional GSHP, a horizontal or vertical collector is used to extract thermal energy from the ground which is transferred to the HP refrigerant through an external working fluid (brine or water) working in a closed loop. However, direct expansion of the refrigerant of the heat pump can also be realised by removing the external closed loop. Low- to mid-temperature geothermal energy (from 10°C to 150°C) can be directly used into distric heating or cooling networks in lieu of fossil fuels. In this case, wells around 2 km of depth can be drilled without affecting the visual landscape.
Geothermal heat pump (GSHP)
Note: BH = borehole.
Source: Bonamente E, Aquino A. Life-Cycle Assessment of an Innovative Ground-Source Heat Pump System with Upstream Thermal Storage. Energies. 2017; 10(11):1854. https://doi.org/10.3390/en10111854
Biomass, waste-to-energy and biofuels [1,3]: local solid biomass can come from waste including municipal solid, wood, agricultural wastes, sewage, and plant material. Waste-to-energy can incentivize waste recycling activities in cities thus promoting circular economy and minimising the decrease in volume of usable waste, which is necessary to ensure a reliable and constant supply of energy. Biomass feedstocks can be burned producing steam to feed turbines that most commonly generate heat and electricity (combined heat and power - CHP). The main direct combustion technologies include mass burning, modular incineration and refuse-drived fuel. Small scale biomass such as wood pellets, briquettes or wood stoves are commonly employed for residential use. In direct combustion applications flue gases and ashes have to be monitored. Biomass can also be converted in combustible oil or biofuels in a gasification process in a low-oxygen environment which can be more efficient compared to the burning of solid biomass (anaerobic digestion). Biogas can be captured from landfills and other facilities such as wastewater and manure treatment plants. The methane produced from the anaerobic digestion can be burned in a combustion process to produce thermal energy.
Biofuel products from biomass
Clauser NM, González G, Mendieta CM, Kruyeniski J, Area MC, Vallejos ME. Biomass Waste as Sustainable Raw Material for Energy and Fuels. Sustainability. 2021; 13(2):794. https://doi.org/10.3390/su13020794
Most technologies used for on-site renewable thermal energy generation are nowadays mature and available on the market.