# The State-of-the-Art of Sensors and Environmental Monitoring Technologies in Buildings
## 4 Battery and Energy Harvesting Technologies for Sensing and Monitoring in Buildings
### Potential of Building Energy Harvesting Technologies  
In 2012, the World Economic Forum's Global Agenda Council on Emerging Technologies highlighted 'Wireless (Energy Harvested) Power' as one of the top 10 technologies that would have the \"greatest impact on the state of the world in 2012\" [111]. The technology offers several unique features which conventional battery technologies cannot always provide, such as self-sustainable capability, prolonged lifetimes, no need for labor-intensive replacements, and easy installations [112]. However, at present, energy harvesting suffers from low, intermittent and unpredictable levels of available power, which limits its application to low-power electronic applications within buildings, e.g., low-power sensors, low-power rechargeable batteries, etc. The natural sources of energy available for harvesting in buildings essentially consist of five main forms [113; 114; 115; 116]:  
1. Photovoltaic (PV)--PV or solar cells convert ambient light energy (either natural sun light or artificial light) into electrical power and are the most mature and commonly used energy harvesting technology today. The amount of power harvested in PV energy harvesting is directly dependent on the incident angle of the light, intensity of the source, and the surface, size, sensitivity and material of the PV cell. 2. Thermal--Thermal (or Thermoelectric) energy harvesters convert heat into electrical power. These harvesters consist of arrays of thermocouples which sense a difference in temperature across their bimetal junctions (Seebeck effect) and produce a voltage in response to that. 3. Kinetic--Kinetic energy harvesters convert ambient mechanical energy (typically vibrations) into electrical power. Most kinetic harvesters, to date, convert vibrations into electrical energy using one of three main mechanisms, i.e., (1) electromagnetic (EM), (2) piezoelectric, and (3) electrostatic. 4. Radio Frequency (RF)--RF energy harvesters convert electromagnetic waves naturally available in the environment into electrical power. These RF waves are typically emitted from devices such as WiFi routers, radio and TV transmitters, mobile communications to and from base stations, etc. 5. Airflow--Airflow energy harvesters use the ambient air generated by air conditioners, etc., inside buildings and convert it into electrical power. This is typically achieved via one of three methods, i.e., (1) wind turbine approach, (2) vibrating ribbon approach, and (3) cantilever approach. To assess the performance of the aforementioned energy harvesting technologies, [PERSON] et al. [115] presented detailed surveys of different residential, commercial office and university buildings in three different European locations; (a) Southampton, UK (Figure 4a,b) San Sebastian, Spain (Figure 4b), and (c) Warsaw, Poland, and published measurements for light, temperature, vibrations, RF and airflow within these buildings. Table 5 provides a summary of the PV, Thermal, Kinetic, RF and Airflow measurements, i.e., estimated electrical dc power, and the calculated power densities of each energy source, for all the above-mentioned locations in [115]. It can be observed that Thermal and Airflow have the highest electrical dc powers (mW range) and power densities (mW/cm\({}^{3}\) range) of all the energy harvesting technologies followed by PV and kinetic (\(\mu\)W range), and finally RF (nW range). However, technologies such as Thermal and Kinetic energy harvesting are heavily reliant on the working of household appliances such as radiators, boilers, washing machines, etc., which does not guarantee a long-term supply of available energy throughout the day.