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Renewable Energy | Technology and Applications 

 

 

Renewable energy sources replenish themselves naturally without being depleted in the earth; they include hydropower, bioenergy, geothermal energy, solar energy, wind energy and ocean (tide and wave) energy and their uses are reported in the table below.

Energy Sources Energy conversion and usage options
 Hydropower  Power generation
 Biomass  Heat and power generation, pyrolysis, gasification, digestion
 Geothermal  Urban heating, power generation, hydrothermal
 Solar  Solar home systems, solar dryers, solar cookers
 Direct Solar  Photovoltaic, thermal power generation, water heaters
 Wind  Power generation, windmills, water pump
 Wave and tide  Barrage, tidal stream

Tab. 1 Renewable energy forms and their uses

These technologies may not be comparable with conventional fuels in terms of production cost, but they could be comparable if we consider their associated externalities, such as their environmental and social effects. Also, it should be noted that economies of scale could play a key role in reducing the unit production cost. Transmission and distribution costs, as well as technologies, do not differ much among the conventional and renewable energies. Below are presented details about the development of the main renewable energy supply technologies with their advantages and disadvantages.

Hydropower

 

 

hydrop

Hydropower is a clean and renewable energy source and it is the most mature and largest source of renewable power.  Considering  the  economic,  technical  and  environmental  benefits of hydropower,  most  countries  give  priority  to  its  development. Developing hydropower is of great importance to alleviate the energy crisis and environmental pollution resulting from the rapid economic growth of China and other countries in the 21st century[5]

The figure 3 shows the general trend of worldwide hydro electricity consumption from 1965 to 2016 and the figure 4 the trend in the more recent years.[6] , [7]

Worldwide hydro electr consum

Fig. 3 Worldwide hydro electricity consumption, 1965-2011

 

Hydrop capacity and addition

Fig. 4 Hydropower Capacity and Additions, Top 9 Countries for Capacity Added, 2016

Hydropower is an essential energy source harnessed from water moving from higher to lower elevation levels, primarily to turn turbines and generate electricity. The primary energy is provided by gravity and the height the water falls down on to the turbine. The potential energy of the stored water is the mass of the water, the gravity factor (g = 9.81 ms−2) and the head defined as the difference between the dam level and the tail water level. The reservoir level to some extent  changes  downwards  when  water  is  released  and  accordingly  influences  electricity  production. Turbines are constructed for an optional flow of water.

Although hydropower plants are highly site-specific (the local topography and hydrology will define the type of facilities that can be built), they can be broadly categorized into three main typologies:

  1. Storage hydropower: a facility that uses a dam to impound river water, which is then stored for release when needed. Electricity is produced by releasing water from the reservoir through operable gates into a turbine, which in turn activates a generator. Storage hydropower can be operated to provide base-load power, as well as peak-load through its ability to be shut down and started up at short notice according to the demands of the system. It can offer enough storage capacity to operate independently of the hydrological inflow for many weeks, or even up to months or years. The primary advantage of hydro facilities with storage capability is their ability to respond to peak load requirements.
  2. Run-of-river hydropower: a facility that channels flowing water from a river through a canal or penstock to drive a turbine. Typically, a run-of-river project will have short term water storage and result in little or no land inundation relative to its natural state. Run-of-river hydro plants provide a continuous supply of electricity, and are generally installed to provide base load power to the electrical grid. These facilities include some flexibility of operation for daily/weekly fluctuations in demand through water flow that is regulated by the facility.
  3. Pumped-storage hydropower: provides peak-load supply, harnessing water which is cycled between a lower and upper reservoir by pumps, which use surplus energy from the system at times of low demand. When electricity demand is high, water is released back to the lower reservoir through turbines to produce electricity. Some pumped-storage projects will also have natural inflow to the upper reservoir which will augment the generation available. Pumped-storage hydropower is practically speaking a zero sum electricity producer. Its value is in the provision of energy storage, enabling peak demand to be met, assuring a guaranteed supply when in combination with other renewables, and other ancillary services to electrical grids. One major advantage of pumped-storage facilities is their synergy with variable renewable energy supply options such as wind and solar power (non-flexible power supply options). This is because pump-storage installations can provide back-up reserve which is immediately usable during periods when the other variable power sources are unavailable.

Although there are clear hydropower typologies, there can be overlap among the above categories. For example, storage projects can involve an element of pumping to supplement the water that flows into the reservoir naturally, and run-of-river projects often provide some level of storage capability. Hydropower technologies are not bound by size constraints, the basic technology is the same irrespective of the size of the development. Large-scale hydropower installations typically require storage reservoirs. Smaller-scale hydropower systems can be attached to a reservoir, or they can be installed in small rivers, streams or in the existing water supply networks, such as drinking water or wastewater networks. Hydropower facilities installed today range in size from less than 100 kW to greater than 22 GW, with individual turbines reaching 1000 MW in capacity. [8]

As the vast stock of hydropower facilities around the world ages, modernization  and  retrofitting  of  existing  facilities  continues  to  be  a  significant  part  of  industry  operations,  with  the  potential  to  increase  greatly  the  performance  of  existing  plants. In addition to ongoing improvements to mechanical equipment such as turbines, plant operators also continue to implement advanced control technologies and data analytics for digitally enhanced power generation. It is expected that these steps will help to optimize plant management for greater reliability, efficiency  and  lower  cost,  while  also  allowing  for  more  flexible  integration  with other grid resources, including variable renewable energy. [9]

 

Bioenergy

Bioenergy is the energy that comes from organic matter, such as plants. Many industries, such as those involved in construction or the processing of agricultural products, can create large quantities of unused or residual biomass, which can serve as a bioenergy source. Many  bioenergy  technologies  and  conversion  processes  are  now  well-established  and  fully  commercial.  A further set of conversion processes, in particular for the production of advanced liquid fuels, is maturing rapidly.

Converting biomass into gas is a process known as gasification. Using gas turbines, these gases can be used to generate electricity. Methane gas produced during the decay of biomass in landfills can also be used to generate electricity or for other industrial processes. Global bio-power capacity increased an estimated 6% in 2016, to 112 GW. Generation rose 6% to 504 terawatt-hours (TWh) . The leading  country  for  electricity  generation  from  biomass  in  2016  was  the  United  States  (68  TWh),  followed  by  China  (54  TWh),  Germany  (52  TWh),  Brazil  (51  TWh),  Japan (38  TWh), India  and  the United Kingdom (both 30 TWh).[10]

 

Global bio-power gener

Fig. 5 Global Bio-Power Generation, by Region, 2006-2016 

 

Biomass can also be converted into a liquid fuel referred to as biofuel through a conversion process. An example of biofuel is ethanol. The current largest source of ethanol is corn. Some cities use ethanol as a gasoline additive to help meet air quality standards. Another example of biofuel is biodiesel, produced from fats of vegetables and animals can be used as fuel for vehicles or as a fuel additive to reduce emissions. In 2016, global biofuels production, which closely tracks demand, increased  around  2%  compared  to  2015,  reaching  135  billion liters.  This  increase  was  due  largely  to  a  rebound  in  biodiesel  production after a decline in 2015. The United States and Brazil remained the largest biofuels producers by far, accounting for 70% of  all  biofuels  between  them,  followed  by  Germany,  Argentina,  China  and  Indonesia. An  estimated  72%  of  biofuel  production  (in  energy  terms)  was  fuel  ethanol,  23%  was  biodiesel, and 4%  was hydrotreated vegetable oil (HVO).  Biomass can also be heated in the absence of oxygen to chemically convert it into a fuel oil called pyrolysis oil. Pyrolysis oil can be used for power generation and as a feedstock for fuels and chemical production. In the following figure there are some conversion methods for biofuels production 10.

 

some conversion path for biofuel

 

Fig. 6 Some conversion Pathways to Advanced Biofuels

 

Geothermal Energy

 

 

Geothermal

The geothermal process involves trapping heat underground and building energy that rises near the surface in the form of heat. When this heat naturally creates hot water or steam, it is harnessed and then used to turn a steam turbine to generate electricity. The Italians were the first to use geothermal energy for commercial purposes in the early 1900’s. Geothermal energy is extremely kind to the environment. It offers a constant, efficient supply of clean energy with minimal impact on its surroundings.[11]

Geothermal energy is created by radioactive decay, with temperatures reaching 4,000˚C at the core of the Earth. While geothermal energy is available worldwide, there is an important factor called the geothermal gradient that indicates whether a region is a favored place for enactment. It measures the rate at which the temperature increases as the depth of the Earth increases. For example, the average geothermal gradient in France is 4˚C/100m with a range of 10˚C/100m in the Alsace region to 2˚C/100m in the Pyrenees Mountains. In Iceland and the volcanic regions, the gradient can reach as high as 30˚C/100m.[12] The geothermal gradient is not the only tool used to measure the accessibility of geothermal energy. The permeability of rocks, which determines the rate of flowing heat to the surface, is considered to be another important measure in the availability of geothermal energy.

Geothermal energy has many different uses that can be grouped into three categories: for generation of electricity, for heating systems (and direct use), and for use in geothermal heat pumps.

Some geothermal plants produce both electricity and thermal output for various heat applications. An  estimated  0.4 GW  of  new  geothermal  power  generating  capacity  came  online  in  2016,  bringing  the  global  total  to  an  estimated  12  GW, as shown in figure 7.[13]

 

cumu geo

 

Fig. 7 Cumulative installed geothermal capacity

 

There are three types of geothermal power plants: Dry steam plants, Flash steam plants and Binary cycle plants. Dry steam plants draw from steam reservoirs, whereas both the flash steam and binary cycle plants draw from hot water reservoirs. Flash steam plants typically use water at temperatures greater than 180 °C. Binary cycle plants transfer heat from the water to a so called working fluid, therefore can operate using water at lower temperatures of about 110°C to 180°C.

Research continued in the field of enhanced (or engineered) geothermal systems (EGS) during 2016, particularly in the United States, where government-funded research has aimed to realize commercial, cost-competitive power production [14]. The common feature  among  all  the  most  productive  geothermal  regions  of  the  world  is  naturally  occurring  hydrothermal  activity,  defined  by the presence of high heat, geothermal fluid and permeability. To  achieve  economical  geothermal  production  elsewhere,  or  to  enhance  production  at  existing  locations,  fracturing  of  sub-surface  rock  formations  can  create  the  needed  permeability  to  form  a  productive  geothermal  reservoir,  which  is  known  as  EGS [15]. In other instances, adequate permeability may exist in hot sedimentary aquifers, but fracturing  may  be  needed  to  ensure  adequate well productivity [16]

 

Solar Energy

 

Solar

 

Solar energy uses the unlimited power of the sun to produce heat, light, and power thus is the most abundant renewable resource on our planet. In spite of this abundance, only 0.04% of the basic power used by humans comes directly from solar sources because using a photovoltaic (PV) panel costs more than burning fossil fuels. Organic materials have recently been intensively studied for PV applications, not because of harvesting the sun’s power more efficiently, but because power generation from organic photovoltaic (OPV) materials will cost considerably less than other PV technologies. [17]

During  2016,  at  least  75  GWdc  of  solar  PV  capacity  was  added  worldwide, equivalent to the installation of more than 31,000 solar panels  every  hour. More  solar  PV  capacity  was  installed  in  2016  (up  48%  over  2015)  than  the  cumulative  world  capacity  five  years  earlier. By  year’s  end,  global  solar  PV  capacity  totalled  at  least  303 GW, as schows the figure 8. For the fourth consecutive year, Asia eclipsed all other markets, accounting  for  about  two-thirds  of  global  additions. The  top  five markets – China, United States, Japan, India and the United Kingdom  –  accounted  for  about  85%  of  additions;  others  in  the  top  10  for  additions  were  Germany,  the  Republic  of  Korea,  Australia, the Philippines and Chile. [18]

solarPV

 

Fig. 8 Solar PV Global Capacity and Annual Additions, 2006-2016

 

While  demand  is  expanding  rapidly  for  off-grid  solar  PV,  the  capacity  of  grid-connected  systems  is  rising  more  quickly  and  continues to account for the vast majority of solar PV installations worldwide. Decentralized (residential, commercial and industrial rooftop systems) grid-connected applications have struggled to maintain  a  roughly  stable  global  market  (in  terms  of  capacity  added  annually)  since  2011,  particularly  with  the  transition  from  FITs and net metering to self-consumption. Centralized large-scale  projects,  by  contrast,  have  comprised  a  rising  share  of  annual installations – particularly in emerging markets – despite grid  connection  challenges,  and  now  represent  the  majority  of  annual  installations. [19]

Innovations   and   advances   continued   during   the   year   in   manufacturing, product efficiency and performance, installation and O&M. They were driven largely by rapid price reductions, which have forced companies to move forward their roadmaps to decrease costs and differentiate themselves. Module  manufacturers  continued  increasing  the  number  of  busbars  to  reduce  internal  electrical  resistance,  as  well  as  reducing  barren  spaces  on  modules  to  enhance  light  trapping [20]. Perovskites achieved further improvements in efficiency and stabilization through ongoing R&D [21]. Efficiency  gains  from  such  advances  have  reduced  the  number  of  modules  required  for  a  given  capacity,  lowering  soft  costs. Labor and other soft costs of large-scale projects also are falling thanks  to  customized  design  testing,  pre-assembly  of  systems  and  advances  in  racking. Inverters  also  are  becoming  more  sophisticated  and  making  a  growing  contribution  to  grid  management,  and  manufacturers  are  working  to  improve  long-term  reliability  and  system-prediction  methods. During 2016, key areas of focus included advancing both materials  and  self-regulating  technologies  in  order  to  build  higher-voltage  central  inverters  and  thereby  reduce  balance  of  systems  costs and the levelised cost of electricity (LCOE), as well as improving performance and software to reduce O&M costs. Efforts   to   advance   recycling   processes   continued,   although   there  was  relatively  small  demand  for  recycling  of  waste  and  solar panels (at end-of-life, or damaged or defective panels) as of 2016 [22]. The cost of new photovoltaic power is dropping rapidly, and if the photovoltaic industry continues to grow and improve technologically, by 2020 the cost could be comparable to the cost of conventional power, as will the cost of solar thermal power. [23]

Concentrated Solar Power

CSP technologies use sun-tracking mirrors to collect and concentrate the sunlight and use it as a form of high-temperature heat for electricity generation and industrial processes. Despite the higher investment cost, operating costs for CSP plants are lower than fossil fuel alternatives, and there are no costs for the “fuel”. Identifying economical reflecting materials could further reduce investment costs [24] .There are three types of CSP systems: power towers (central receivers), parabolic troughs, and dish/engine systems. This also has low operating costs and high efficiency, and can produce a reliable supply of energy by utilizing thermal storage [25]. Currently glass mirrors lined with silver are the most efficient material for CSP (95%), yet due to their high cost, weight, fragility and risk of corrosion from dust storms, CSP developers and operators are seeking alternatives. Two parameters were defined to assess the quality of all types of solar reflector materials: reflectance and specularity (capacity to reflect all light into the direction of the solar receiver). Four commercially available materials (floating glass mirrors, metalised polymer films, polished aluminium and anodised aluminium) were tested. The best optical performance was achieved by glass mirrors lined with silver as they presented the highest solar reflectance and optimum specular behavior 24 .

 

Wind Energy

 

wind2

 

Wind energy has been the fastest growing source of energy in the world since 1990. Wind power is a very simple process. A wind turbine converts the kinetic energy (motion) of wind into mechanical energy that is used to generate electricity. The energy is fed through a generator, converted a second time into electrical energy, and then fed into the grid to be transmitted to a power station. Wind turbines are highly sophisticated power systems that capture the wind’s energy by means of new blade designs or airfoils.

Almost  55  GW  of  wind  power  capacity  was  added  during  2016,  increasing  the  global  total  about  12%  to  nearly  487  GW. Gross  additions  were  14%  below  the  record  high  in  2015,  but  they  represented  the  second  largest  annual  market  to  date  as shown in figure 9. By the end of 2016, over 90 countries had seen commercial wind power activity, and 29 countries – representing every region – had more than 1 GW in operation. A significant decline in the Chinese market (following a very strong 2015) was responsible for most of the market contraction. Even so, China retained its lead for new installations, followed distantly by  the  United  States  and  Germany,  with  India  passing  Brazil  to  rank  fourth [26].

 

wind power capacity

Fig. 9 Wind Power Global Capacity and Annual Additions, 2006-2016

 

 

Ocean Energy

 

Ocean

 

Extracting energy from the ocean is considered to be an interesting option, due in part to the wide availability of ocean sources. Surface waves are created when wind passes over water. The faster the wind speed, the longer the wind is sustained, the greater distance the wind travels, the greater the wave height, and the greater the wave energy produced. Ocean energy refers to any energy harnessed from the ocean by means  of  ocean  waves,  tidal  range  (rise  and  fall),  tidal  streams,  ocean  (permanent)  currents,  temperature  gradients  and  salinity  gradients.

Ocean thermal energy can be used for many applications, including electricity generation. Electricity is generated by using either the warm surface water or boiling the seawater to turn a turbine, which starts a generator. Using tidal and wave energy to produce electricity usually involves mechanical devices. A dam is typically used to convert tidal energy into electricity by forcing the water through turbines. Meanwhile, wave energy uses mechanical power to directly start a generator, to produce electricity [27].

Few commercial ocean energy facilities have been built to date. Of  the  approximately  500  MW  of  operating  capacity  at  the  end  of  2016, see figure 10.[28]

cumulaive marine

Fig. 10 Cumulative installed marine energy capacity

A  great  number  of  research  and  development  (R&D)  projects  is  under  way  in  a  growing  number  of  countries,  with  several  new  deployments  of  ocean  energy  devices  in  2016 .  Most  of  the  projects  focus  on  tidal  stream  and  wave  energy,  but  some  active  projects  also  exist  in  the  areas  of  thermal  and  salinity gradients. To accommodate R&D, ocean energy test centers are proliferating  around  the  world,  often  with  the  active  support  of  local governments . As of late 2016, projects were under way in Canada,  Chile,  China,  the  Republic  of  Korea,  the  United  States  and several countries in Europe [29].

It is interesting to note that despite several tidal devices still operating to date, and the first tidal arrays likely to be built in 2015, many wave energy companies have fared badly. Companies such as Pelamis (considered at one point to be the leaders in the sector) have been taken into administration. The reasons for such failures are not clear, though installing devices in areas of high wave energy is likely to pose greater challenges than was initially thought. Given the uncertainties and harsh environment in which wave devices have to operate, the regular nature of tidal currents appears to make this type of technology more predictable and easier to maintain and operate than waves. [30] .

 

Advantages and Disadvantages

In the table below advantages and disadvantages for renewable energy sources is reported.

Energy Sources PROS CONS
 Hydropower
  • no particulate pollution
  • capable of storing energy for many hours
  • high level of reliability
  • proven technology
  • high efficiency
  • very low operating and maintenance costs
  • ability to easily adjust to load changes
  • can affect wildlife habitats
  • can affect the water quality
  • high initial costs of facilities
  • dependence on precipitation
 Biomass
  • contains less sulphur than coal
  • considered part of the terrestrial carbon cycle
  • insufficient source of energy compared to fossil fuels
  • removal of the green vegetation
Geothermal
  • sustainable and safe for the environment
  • renewable, abundant, and reliable energy source
  • low emission
  • rarity of suitable geothermal power plant locations
  • safety concerns due to hazardous materials from underground
  • produced energy is difficult to transport
 Solar
  •  does not create greenhouse gases
  •  no noise
  •  no moving parts
  •  require very little maintenance
  •  maintenance and repair costs very   reasonable
  •  solar panels production is almost expensive
  •  required to store the energy for use
  •  cannot be collected at night
  •  dependent on weather conditions
 Wind
  • free, abundant, and sustainable energy
  • located onshore (land) or offshore
  • fully cost-competitive
  •  far from population centres
  •  intermittent and unpredictable nature
  •  environmental constrains
  •  interference with radio and TV signals
  •  interfering with migratory birds
 Wave and Tide
  • most appropriate source of energy for small island states
  • quick reduction of costs in next future
  • at the initial stage of commercialization
  • expensive
  • time and season dependant
  • impact coastal ecosystems or habitats

Tab. 2 Pros e Cons for renewable sources of energy


[5] Huang, Hailun, Yan, Zheng, 2009. Renewable & Sustainable Energy Reviews, 13 (6/7):1652-1656

[6] A Review of Renewable Energy Supply and Energy Efficiency Technologies, IZA DP No. 8145

[7] Førsund, F. R. (2015). Hydropower economics (Vol. 217). New York: Springer

[8] https://www.worldenergy.org/wpcontent/uploads/2017/03/WEResources_Hydropower_2016.pdf

[9] https://www.ge.com/digital/sites/default/files/Power%20Digital%20Solutions%20Product%20Catalog.pdf.

[10] http://www.ren21.net/wpcontent/uploads/2017/06/178399_GSR_2017_Full_Report_0621_Opt.pdf

[11] http://www.geothermalengineering.co.uk/

[12] Ngô, C., & Natowitz, J. (2009). Our energy future: resources, alternatives and the environment (Vol.11): Wiley.

[13] https://ourworldindata.org/renewables#geothermal

[14] https://www.energy.gov/sites/prod/files/2017/03/f34/GTO%202016%20Annual%20Report_1.pdf.83

[15] https://energy.gov/sites/prod/files/2016/05/f31/EGS%20Fact%20Sheet%20May%202016.pdf

[16] https://setis.ec.europa.eu/sites/default/files/reports/2015_jrc_geothermal_energy_status_report.pdf.85Ibid.239ff

[17] Moule, A., 2010. Current Opinion in Solid State & Materials Science, 14 (6):123-130.

[18] Førsund, F. R. (2015). Hydropower economics (Vol. 217). New York: Springer

[19]Residential markets are located primarily in Australia, several countries in the EU, Japan and the United States. In 2015, the global solar rooftop segment declined by 1 GW relative to 2014

[20] http://www.photovoltaic-conference.com/press-media/blog-2017/118-blog-2017-170104.html.1

[21]http://www.kit.edu/kit/english/pi_2016_133_record-for-perovskite-cigs-tandem-solar-module.php

[22] http://www.ren21.net/wpcontent/uploads/2017/06/178399_GSR_2017_Full_Report_0621_Opt.pdf

[23] Delucchi, M., and Jacobson, M. 2013. Providing all global energy with wind, water, and solar power.Energy Policy, 39(3): 1170-1190

[24] https://www.iea.org/tcp/renewables/solarpaces

[25] Bull, SR., 2001. Renewable energy today and tomorrow. Environmental Sciences and Pollution Management: Proceedings of the IEEE, 89(8): 1216-1226

[26] http://www.ren21.net/wpcontent/uploads/2017/06/178399_GSR_2017_Full_Report_0621_Opt.pdf

[27] https://www.azocleantech.com/article.aspx?ArticleID=11

[28] https://ourworldindata.org/renewables#wave-tidal

[29] https://hendryreview.files.wordpress.com/2016/08/hendryreview-final-report-english-version.pdf.

[30] Recent Developments in Ocean Energy and Offshore Wind: Financial Challenges and Environmental Misconceptions, Miguel Esteban1, Alexandros Gasparatos2, Christopher N.H. Doll3, 2017