Batteries are the main source of energy for low-power electronics such as micro-electro mechanical systems (MEMS), wireless sensor networks, embedded devices for remote sensing and control, etc. With the limited capacity of finite power sources and the need for supplying energy for the lifetime of a system/device there is a requirement for self-powered devices. Using conventional batteries is not always good design solution because batteries require human intervention to replace them (very often in hard-accessible and harsh-environmental conditions). Therefore, acquiring the electrical power, by using an alternative source of energy that is needed to operate these devices is a major concern. The process of extracting energy from the surrounding environment and converting it into consumable electrical energy is known as energy harvesting or power scavenging. The energy harvesting sources can be used to increase the lifetime and capability of the devices by either replacing or augmenting the battery usage. There are various forms of energy that can be scavenged, like solar, mechanical, thermal, and electromagnetic. Nowadays, there is a big interest in the field of research related to energy harvesting. This paper represents a survey for identifying the sources of energy harvesting and describes the basic operation of principles of the most common energy harvester. As first, we present, in short, the conversion principles of single energy source harvesting systems and point to their benefits and limitations in their usage. After that, hybrid structures of energy harvesters which simultaneously combine scavenged power from different ambient sources (solar, thermoelectric, electromagnetic), with aim to support higher load at the output, are considered
