2.3.3.5 Gravity Hydropower Converters
Gravity hydropower converters include hydrodynamic screws and gravity water wheels (Quaranta and Revelli 2018). Gravity water wheels are classified into three types (based on the head differences and the maximum flow rate per metre width), namely overshot, breastshot and undershot. Efficiency of gravity water wheels ranges between 75 and 85% (Quaranta and Revelli 2018), and their cost is lower than that of Kaplan turbines and hydrodynamic screws. With improved designs owing to recent developments, these hydropower converters are becoming cost‐effective and the effect on fish migration is decreasing along with better efficiency. Installation of these water wheels at old mill sites and in canals leads to electricity production which makes them more profitable (Quaranta and Revelli 2018). The hydrostatic pressure machine (HPM) and the turbine water wheel (TWW) are the examples of the novel water wheels which are in their primary stage. Hydraulic efficiency of HPM is 60–65% with an advantage of using running water without any canal drop (Kougias et al. 2019). The TWW finds its usage in head differences (up to 6 m) of an overshot water wheel and flow rates (few m3s−1) of an undershot water wheel (Helmizar 2016).
2.3.3.6 Pump as Turbines (PAT)
When a hydraulic pump operates in reverse mode i.e. as a turbine, it is termed as PAT. With the help of a connected induction motor which works as a generator, PAT generates electricity (Agarwal 2012) opposite to what a pump does. Worldwide, pumps are produced at a massive scale, so if they are used as turbines, then it comes with many benefits such as reduced dimensions, simple maintenance along with easy availability of spare parts. PAT is 5–10 times cheaper than the conventional turbines along with low installation cost (Novara et al. 2017). Using PAT in small hydro plants having installed capacity <100 kW makes them cost‐effective as the cost of a turbo‐generator unit is normally 35% of the total cost of the plant (Ogayar and Vidal 2009). Along with many advantages over conventional turbine, PATs have some disadvantages also, such as lower/lesser peak efficiency, design and operation uncertainties and lack of in‐built regulation devices. At the ground level, contribution of PATs in the market is very small partially due to lack of information or awareness from its manufactures and hydropower consulting firms (Kougias et al. 2019). PATs find its significant application in rural and remote off‐grid power generation projects where conventional turbines cannot be used due to high cost and low availability (Agarwal 2012).
In addition to the evolution in technology in small‐scale hydropower plants to harness more energy, there is an opportunity of development in existing small dams. Some small dams, established in rural areas, are non‐powered dams as the aim was to fulfil other needs of rural areas such as irrigation and drinking water supply (Kougias et al. 2014). Thus, the transformation of small dams to small hydropower plants provides a good opportunity for power generation along with the reduction in cost and saving time.
2.3.3.7 Developments in Fish‐Friendly Hydropower
Hydropower plants have some adverse impacts on the environment and preservation of ecosystems. Various studies have been conducted to understand the effect of hydro plants on aquatic species populations, especially fishes. In preservation of ecosystems, downstream and upstream migration of fishes is very significant. Recent advances in the development of fish‐friendly turbines along with progress in water‐lubricated bearings (used in turbines) have been reported by several research projects (Hogan et al. 2014; Quaranta et al. 2017; Fjeldstad et al. 2018). Gravity water wheels and hydrodynamic screws which come under low head gravity turbines are fish‐friendly as opposed to conventional turbines. However, these turbines can be used in small hydro plants (very low head sites). For the large‐scale hydropower plant, two different approaches have emerged to allow fish migration, namely fishways facilities and usage of fish‐friendly turbines (Kougias et al. 2019).
Conventional turbines hinder the migration of fishes (upstream and downstream) so to allow the migration, hydraulic structures are used, termed as fishways (Quaranta et al. 2017). For making eco‐efficient fishways having optimal design, more studies are needed to understand their swimming ability and at what position inlet or outlet should be installed so that fishes can use them efficiently (Fjeldstad et al. 2018).
In the high head hydropower plants, fishways find some restrictions, so in order to overcome these restrictions, some advances are taking place in the turbine designs. For safe passage of fishes through hydraulic turbines, a technology termed as fish‐friendly turbine has emerged, owing to the developments in the area of advances in turbines. Recently, two different turbines with better designs have been introduced, the Alden turbine and the Minimum Gap Runner turbine. Usage of these turbines leads to more power generation, reduction in fish mortality rate and injury along with better downstream water quality (Hogan et al. 2014).
2.3.4 Geothermal Energy
Geothermal energy is generated within the sub‐surface of earth in the form of heat. Direct use of this energy is for heating purposes (Lund and Boyd 2016), and/or it can also be harnessed for electricity generation using a geothermal power plant (Bertani 2016). Compared with the intermittent sources of energy (solar and wind), geothermal energy is constant around the year and it is available worldwide. Electricity generation from the geothermal energy comes with many benefits as compared with some other renewable energy sources such as higher capacity factors (>90%), flexibility in power production, ability of efficiently providing baseload electricity, low cost of electricity generation due to lower cost of operation, lower impact on environment along with less CO2 emissions and land usage is also small (Geirdal et al. 2015). Electricity production from geothermal has lower life‐cycle greenhouse gas (GHG) emissions as compared with fossil fuels (https://archive.ipcc.ch/pdf/special‐reports).
Despite many advantages of geothermal energy, its share in worldwide energy production is very small owing to many challenges such as substantial initial capital cost, longer time period before a geothermal plant becomes operational and high risk at early stage (pre‐survey, exploration and test drilling) (IRENA 2017). As mentioned earlier in Section 3, contribution of geothermal energy was 14 GW by the end of 2019 which is a relatively small percentage (<0.6%) of the total generation capacity of all the renewable sources of energy.
Geothermal energy has tremendous potential for development in near future owing to the huge amount of energy present within 10 km of the earth's surface which contains 50000 times more energy than all the fossil fuel reserves existing globally (Shere 2013, pp. 201). Depending upon the amount of heat present in a geothermal reservoir, different technologies are used for power generation. Medium or high‐temperature resources are required for power production which are generally located in the vicinity of tectonically active regions. To transfer the heat present within the earth's surface, water or steam is used. At present, four different technologies are used for power production from geothermal energy, namely dry steam plants, flash plants (single, double and triple), binary plants, and combined‐cycle or hybrid plants (Long et al. 2003). These technologies are based on exploitation of conventional geothermal resources.
Figure 2.3 Comparison of geothermal power capacity (MW) by different countries in 2016 and projected values in 2025 and beyond 2025.
Source: Based on data in Ref. (IRENA 2017).
Recently, due to more developments in enhanced geothermal systems (EGS), which are the deep geothermal resources, power production from these types of plants will add a new dimension in the geothermal energy sector. Power generation will increase by many folds and can reach to 70 GWe (installed capacity) by 2050 (Lu 2018). Figure 2.3 shows the geothermal power capacity of