Geothermal technologies differ in the type of resource that they use for power or heat generation. Three types of resources are: high-temperature hydrothermal resources (volcanic resources), low and medium-temperature hydrothermal resources, and hot rock.
Flash steam plants, which make up about two-thirds of geothermal installed capacity today, are used where water-dominated reservoirs have temperatures above 180°C. In these high-temperature reservoirs, the liquid water component boils, or “flashes,” as pressure drops. Separated steam is piped to a turbine to generate electricity and the remaining hot water may be flashed again twice (double flash plant) or three times (triple flash) at progressively lower pressures and temperatures, to obtain more steam.
Dry steam plants, which make up about a quarter of geothermal capacity today, directly utilise dry steam that is piped from production wells to the plant and then to the turbine. Control of steam flow to meet electricity demand fluctuations is easier than in flash steam plants, where continuous up-flow in the wells is required to avoid gravity collapse of the liquid phase.
Binary plants constitute the fastest-growing group of geothermal plants, because they are able to also use the low- to medium-temperature resources, which are more prevalent. Binary plants, using an organic Rankine cycle (ORC) or a Kalina cycle, typically operate with temperatures varying from as low as 73°C (at Chena Hot Springs, Alaska) to 180°C. In these plants, heat is recovered from the geothermal fluid using heat exchangers to vaporise an organic fluid with a low boiling point (e.g. butane or pentane in the ORC cycle and an ammonia-water mixture in the Kalina cycle), and drive a turbine. Today, binary plants have an 11% share of the installed global generating capacity and a 44% share in terms of the number of plants.
Geothermal energy can also provide heat. Even geothermal resources at temperatures of 20°C to 30°C (e.g. flood water in abandoned mines) may be useful to meet space heating demand or other low-temperature applications. Geothermal “heat-only” plants can feed a district heating system, as can the hot water remaining from electricity generation, which can also be used in applications demanding successively lower temperatures. Because transport of heat has limitations, geothermal heat can only be used where a demand exists in the vicinity of the resource
Geothermal technologies using hot rock resources could potentially enable geothermal energy to make a much larger contribution to world energy supply. Technologies that utilize hot rock resources are also known as enhanced or engineered geothermal systems (EGS). These systems aim at using the earth’s heat where no or insufficient steam/hot water is available or where permeability is low. EGS plants differ from conventional plants only as far as heat/steam extraction is concerned. EGS technology, therefore, is centred on engineering and creating large heat exchange areas in hot rock. The process involves enhancing permeability by opening pre-existing fractures and/or creating new fractures.
Conventional geothermal is a mature technology that can provide baseload power or year-round supply of heat. The resource can be exploited only in favourable regions (a constraint that can be relaxed when EGS systems are ready to be commercialised). Matching heat demand to resource availability can be difficult given the costs and difficulty of transporting heat long distances.