Renewable Energy Sources (RES) such as biomass, wind, solar energy, hydropower, etc, can produce sustainable energy. Renewable energy systems are usually developed in small-scale.
Some renewable energy technologies are hereby introduced, as example of what can be applied in public and private buildings / facilities in order to reduce electricity costs and the use of fossil fuel.
1. Wind Energy
Electricity produced by a wind turbine highly depends on the annual average wind speed at the site. The higher the wind speed, the higher the quantity of energy produced. The actual amount of energy generated depends by the amount of energy in the wind passing through the area swept by the turbine blades in a unit of time.
A device that generates electricity from wind is referred to as wind turbine (WT). A wind turbine uses kinetic energy of the wind flow. The rotors in the turbine reduce the wind velocity from the undisturbed wind speed
In general, most of small/micro wind turbines start to generate electricity with a wind speed of 3-4 m/s. The generation of energy varies according to wind variations in the area, although the most rapid variations would to some extent be compensated by the inertia of the wind turbine rotor.
Wind speed increases with height, as well as closeness to the ground and objects disrupts air flow. Therefore, the turbines shall be kept somewhere free from obstacles / obstructions (such as buildings or trees).
A wind turbine should not be located where there is very turbulent air flow. Indeed, excessive turbulence may cause fatigue and shorten the turbine’s life.
As general rule, the bottom of the turbine’s blades should be located at least 3 meters above the top of anything within 100 meters.
Wind turbines designs can be categorized in horizontal or vertical axis turbines
Horizontal axis wind turbines (HAWT) typically have two or three blades (small HAWT may have more blades). A 3-blade turbine is the most energy efficient one and, due to this particularity, the most commercialized.
Vertical axis wind turbines (VAWT), up to now, are more expensive than HAWT, and therefore not so commercialized. However, VAWT are still produced for remote power applications. Recent research has shown that VAWT are more suitable for building mounted applications, as they are less affected by turbulence than HAWT.
Small turbines are mainly used for remote power generation, either alone or in conjunction with other energy sources and battery storage:
- Stand – alone system: small / micro wind turbines have traditionally been used to generate energy for charging batteries to run small stand – alone electrical applications, often in remote locations such as farms, island communities, boats, etc., basically where it is not possible to connect to a main power supply.
- Grid-connected system: this installation sends the generated electricity to a public grid. They can either feed the energy need of a house, a farm, or any type of consumption point.
- Hybrid system: it consists in an installation that, apart from the wind speed, generates energy using also other systems (i.e. PV, biomass, etc.).
2. Heat Pumps
Heat naturally flows from high temperature to low temperature. Heat pumps are able to force heat to flow in the other direction using a relatively small amount of high quality drive energy (electric of mechanical energy or high temperature waste heat). Heat pumps can extract heat from various low temperature sources: air, water, industrial or domestic waste, etc.) and transfer it to the heating system
of a building or industrial application at a higher temperature. By reversing the cycle, heat pumps can also be used for cooling.
The majority of heat pumps for domestic applications are built around electrically driven refrigerant compressors.
The heat delivered by a heat pump is theoretically the sum of the heat extracted from the heat source and the energy needed to drive the cycle.
3. New Solar Technologies
While the main function of a traditional PV system is the efficient generation of energy, the new PV systems, to be integrated in the building envelope, shall satisfy a number of additional requirements, such as appearances (i.e. color, size…), weather – tightness, wind loading, durability and maintenance, safety (fire, electrical and structure stability), etc.
At the moment, there are several possible PV integration options:
- Shading system: including louvers, either horizontal or vertical, which may be mounted on the outdoor envelope, over windows or balconies. The function of this system (other than the generation of energy from solar radiation) is to shade the building from excessive light.
- Rain-screen system: cladding panels can be used to protect the load – bearing external walls from rain water. PV panels can both generate energy during sunny days and be a protection from rain during rainy days. This system is usually positioned vertically. The ventilated cavity between the exterior cladding and the main wall contribute to keep down the operating temperature of PV cells, enhancing their performance.
- Stick – system curtain walls: curtain walls are used in buildings with internal columns or structural steel to support the main loads. Curtain walls are not weight bearing and their main function is to resist air and moisture infiltration. Building insulation is often integrated (warm façade). PV panels can be integrated into curtain walls, substituting the classic glass.
- Unitized curtain walls: the unitized wall segments are preassembled. The controlled industrial environment ensures more precision and quality for sealing the PV panels into the envelope.
- Double – skin façade: glass facades are often designed as “double – skin”, when there is significant air space between the internal and external glass walls. This helps reducing the heat transfer losses through the walls. PV cells can be easily integrated into the external façade. Ventilation through the double – skin structure provides valuable cooling on the back of the panel.
- Atria and canopies: in order to achieve the highest performance, PV systems shall be installed horizontally. They can be installed on tilted elements, such as atria or canopies. These structures are usually free from overshadowing and easy to ventilate. However, these structures may also be more affected by heat transfer losses.
- Solar windows (Photovoltaic transparent glass): PV glass can be used as building material, substituting the classic window glass, in order to capture the sunlight and transform it into electricity.