# The State-of-the-Art of Sensors and Environmental Monitoring Technologies in Buildings
## 6 Summary and Conclusions  
In this paper, we have presented an in-depth review of the state-of-the-art of different sensing and environmental monitoring technologies commercially available and widely used today for building applications. By surveying published literature, manufacturer datasheets and user manuals available at the time of publication, we (1) identify the most essential and prominent sensor technologies as: temperature, carbon dioxide, humidity, occupancy, light and airflow measurement technologies, and communication technologies and protocols (both wired and wireless) as: Ethernet, PLCC, Serial Communications, Modbus, BACnet, Zigbee, Bluetooth, BLE, WiFi, EnOcean, 6 LoWPAN, Z-Wave and LoRaWAN, and (2) provide a detailed insight into their operating principles and key characteristic features, such as accuracies, measurement ranges, response times, portability, power consumptions, market adoption and costs. In addition to the detailed survey of various sensor and communication technologies, we also (1) identify the most prominent battery technologies used for sensing and monitoring purposes as: Nickel Cadmium, Nickel Metal Hydride, Nickel Iron, Lead acid, Lithium-ion and Reusable Alkaline, (2) review their key characteristics, such as cycle life, lifespans, self-discharge rates, charge times, environmental impact, maintenance requirements, costs, and power densities, and (3) highlight various time-related limitations of battery technologies, such as self-discharging, size, cost, installation, disposal, maintenance and the eventual need for replacement. In the last decade, energy harvesting from ambient sources within buildings has also emerged as a leading candidate to address the limitations of battery technologies and use the harvested electrical energy to power small low-power electronic devices. We (1) identify the five main sources for energy harvesting in buildings as: Photovoltaic, Thermal, Kinetic, Radio Frequency and Airflow, (2) highlight the potential of each of these sources by discussing the findings of a real-world building case-study conducted in three different European countries, i.e., UK, Spain and Poland, and (3) review various commercial off-the-shelf energy harvesting solutions available for building applications. The literature suggests that power densities generated from building energy harvesting are significantly lower than those available from various battery technologies. However, these power densities (in the mW range) can be sufficient to power low-power sensors and monitoring devices, and recharge batteries used in buildings. In the final section of this review, we discuss different aspects regarding the application of sensing and monitoring technologies within buildings, and various associated challenges and future perspectives. The selection of the most suitable technologies can be a challenging task as there are a number of parameters that building designers, owners, managers, and occupants must take into consideration. We highlight 13 key parameters that must be taken into account, and review proposed strategies and topologies for the optimal placement of sensor devices within buildings. Moreover, we discuss key topics such as routing protocols for energy efficiency enhancement, data analysis and network security and the future of building IOT and energy harvesting technologies. The unique features that energy harvesting provides such as self-sustainable capability, prolonged lifetimes, no need for labor-intensive replacements, easy installation, and the variety of available natural sources of energy within buildings, makes energy harvesting one of the most attractive technologies for buildings of the future. Conceptualization, H.H. and R.P.L. ; Writing--Original draft preparation, H.H. ; Writing--Review and Editing, H.H., T.G., D.B., R.P.L., M.B., C.W., B.P., and J.R.S. This work was supported by the EPSRC (EP/N020863/1), Innovate UK, and the European Regional Development Fund through the Welsh Government (ERDF 80892). The authors declare no conflict of interest.