Renewable Energy: Driving Green Construction Toward a Sustainable Future

Renewable energy is driving green construction toward a sustainable future, with solar photovoltaic energy being the technology with the highest penetration in the building sector. The cost of photovoltaic modules has dropped by 99% since 1976 (from 76 USD/W to 0.20-0.30 USD/W in 2024, IRENA), with module efficiencies of 20-23% for monocrystalline silicon (compared to 6% in 1976). A 10 kWp rooftop self-consumption photovoltaic system (50-60 m2) generates 13,000-16,000 kWh/year in climate zone IV (southern Spain) and 10,000-12,000 kWh/year in climate zone D (northern plateau), with an installed cost of 0.8-1.2 EUR/Wp (8,000-12,000 EUR total) and a return on investment of 4-7 years (RD 244/2019 on self-consumption).

BIPV (Building Integrated Photovoltaics) integrates photovoltaic cells into the constructive elements of the building envelope: solar roof tiles (Tesla Solar Roof, Autarq: 1,200-1,800 EUR/m2, efficiency 15-19%), photovoltaic facade glazing (Onyx Solar, Polysolar: 10-40% transparency, 5-10% efficiency, adjustable g-value), photovoltaic louvres for solar protection (dual function: shading + generation), and photovoltaic rainscreen cladding for ventilated facades (3-6 mm thick, 7-12 kg/m2). The CTE DB-HE5 (Minimum renewable energy contribution) requires a solar photovoltaic contribution for non-residential buildings with floor area exceeding 3,000 m2, calculated through a power-to-area ratio. In Europe, the EPBD 2024/1275 (recast) requires that all new buildings incorporate rooftop solar energy by 2030.

Air-source heat pumps (air-to-water and air-to-air) represent the reference HVAC technology for sustainable buildings, with a COP (Coefficient of Performance) of 3.5-5.0 in heating mode (EN 14511, at 7 degrees C outdoor / 35 degrees C supply) and an EER of 3.0-4.5 in cooling mode (at 35 degrees C outdoor / 7 degrees C supply). This means that for every kWh of electricity consumed, the heat pump delivers 3.5-5.0 kWh of heat, equivalent to an efficiency of 350-500% compared to 90-95% for a gas condensing boiler.

Air-to-water heat pumps using R-290 (propane) or R-32 refrigerant (low GWP: 3 and 675 respectively, compared to 2,088 for R-410A) represent the current trend in compliance with the F-Gas Regulation (EU 2024/573) that progressively phases out high-GWP refrigerants. The cost of a 10 kW air-to-water heat pump for a dwelling is 6,000-12,000 EUR (installation included), with 50-70% savings on the heating bill compared to natural gas and a payback period of 4-8 years. Manufacturers such as Vaillant (aroTHERM plus), Daikin (Altherma 3), and Saunier Duval (GeniaAir Max) offer models with COP exceeding 5.0 at 7 degrees C and seasonal SCOP of 4.5-5.2 (EN 14825). Spain's PNIEC 2021-2030 plans the installation of 3.5 million heat pumps in the residential sector by 2030.

Low-enthalpy geothermal energy (harnessing the stable subsurface temperature: 12-18 degrees C from 10 m depth in Spain) offers the highest performance among HVAC technologies: COP of 4.5-6.0 in heating and EER of 5.0-7.0 in cooling (EN 15450). The capture systems are: vertical borehole heat exchangers (boreholes of 80-200 m with HDPE pipes, extraction capacity of 40-80 W/m depending on ground type), horizontal ground loops (pipes buried at 1.5-2.0 m, 20-40 W/m2, requiring 1.5-3 times the floor area to be conditioned), and energy piles (heat exchange pipes integrated into foundation piles: 20-50 W/m).

The cost of a geothermal system for a 2,000 m2 office building is 80,000-150,000 EUR (drilling + heat pump + distribution), 50-100% higher than an equivalent air-source system, but with an additional 30-40% annual energy saving (thanks to the higher COP and stability of the thermal source). The return on investment is 8-15 years, but the service life of geothermal boreholes is 50-100 years (no moving parts and no maintenance). In Spain, the IGME (Instituto Geologico y Minero) has published the Map of Geothermal Resources of Spain (2020) identifying the zones with greatest potential. The Torre Bolueta building (Bilbao, 2018, certified as the world's tallest Passivhaus: 88 m, 171 dwellings) uses geothermal energy with 200 m boreholes as its primary heating system.

Solar thermal energy heats water through solar collectors (flat plate or evacuated tube) for domestic hot water (DHW), low-temperature heating (underfloor heating), and solar cooling (absorption chillers). The CTE DB-HE4 requires a minimum solar contribution for DHW of 30-70% depending on climate zone and demand, with values of 50-70% in zones IV and V (southern Spain). A 2.5 m2 flat plate solar collector (optical efficiency eta-0 = 0.75-0.80, heat loss coefficient a1 = 3.5-4.5 W/m2K) produces 800-1,200 kWh/year in Spain, covering 60-80% of the DHW demand for a 4-person household.

Biomass (ENplus A1 certified wood pellets, NCV = 4.7-5.0 kWh/kg, biogenic CO2 emissions: neutral over the life cycle) is a renewable alternative for heating in areas without natural gas access. Pellet boilers achieve efficiencies of 90-95% (EN 303-5 class 5) with particulate emissions below 20 mg/Nm3 (complying with the Ecodesign Directive 2015/1189). The cost of pellets in Spain is 0.04-0.06 EUR/kWh (2024), compared to 0.08-0.12 EUR/kWh for natural gas, providing a 30-50% saving in energy costs. However, biomass in urban areas with air quality issues (PM2.5) is increasingly restricted: Barcelona, Madrid, and other cities limit the installation of biomass boilers in atmospheric protection zones.

Small wind energy (turbines rated below 100 kW) has a specific niche in buildings: locations with average wind speeds exceeding 5 m/s at hub height (12-month measurement required). Vertical axis wind turbines (VAWT: Darrieus, Savonius) are better suited to urban environments due to their lower sensitivity to turbulence and variable wind direction. A 5 kW rooftop turbine (average wind speed 6 m/s) generates 5,000-8,000 kWh/year, with an installed cost of 15,000-25,000 EUR and a payback period of 8-15 years. However, the variability of the urban wind resource (turbulence, effect of neighboring buildings) reduces actual production by 30-50% compared to theoretical estimates.

Hybrid renewable systems maximize energy self-sufficiency: the combination of photovoltaics + heat pump + battery is the most widespread in residential buildings. A system comprising 10 kWp photovoltaic + 8 kW heat pump + 10 kWh battery achieves a self-consumption rate of 60-80% and a self-sufficiency rate of 40-60% in a single-family dwelling in Spain (depending on the consumption profile). In office buildings, the combination of BIPV + geothermal + heat recovery enables the nZEB standard to be met (non-renewable primary energy consumption below 60-90 kWh/m2 per year depending on climate zone, EPBD). The Powerhouse Brattorkaia building (Trondheim, Norway, 2019, Snohetta) is an energy-positive building: it generates 20% more energy than it consumes over its 60-year life cycle, thanks to 3,000 m2 of photovoltaics on the facade and roof combined with geothermal boreholes reaching 300 m depth.