Finland is considered weak country to invest in solar energy technologies, but the annual difference in irradiation compared to Germany is not that high. The real reason behind solar energy’s success in Germany is their well-executed feed-in tariff system. The financial support has made solar energy production prices to approach typical customer electricity prices, which has led to lowered PV-equipment prices and easier access for typical consumer to use solar energy. (Pasonen et al. 2012, 61-68.) In Finland PV-panel market does not reflect global market prices and the panel may cost over two times more than in Germany. In addition to expensive equipment, solar energy usage is not highly supported with tariffs. (Pasonen et al. 2012, 21-22.) Regardless of Germany’s knowledge that utilizable solar radiation is low, it has financially supported growth of solar energy technologies and raised their nation to be European leader in renewable energy systems.
7 CONCLUSIONS
Intermittent sources of energy are uncertain by nature causing unstable energy production and difficulties in output forecasting. The main problems incurred by intermittency are operational difficulties, unexpected imbalance between energy demand and supply, and lowered power quality. However, these drawbacks are solvable with the energy storage solutions, redesigning of the grid, demand management, and capacity redundancy.
Currently, integration of the intermittent energy sources is mainly provided by using hydropower and fossil-fuels as an adjusting power, but in the future, demand management and storage technologies will have the major role in intermittency mitigation.
From renewable energy sources, solar, wind, tidal and wave energy are considered intermittent. Solar energy has seasonal and diurnal fluctuations with added irregularities by clouds and aerosols, but its energy production is moderately predictable. The intermittency of wind energy is a more complex process, as its energy production may vary annually, seasonally and diurnally, coupled with highly variable turbulence and topographical elements. Tidal energy has the most predictable intermittency, as it follows semidiurnal pattern, with minor irregularities caused by low pressure and wind. The intermittency of wave energy is based on the same reasons as wind. In conclusion, solar and tidal energy have more predictable intermittency, but they do not produce energy at certain times of the day or at certain months, whereas wind and wave energy have higher variations, but are not so dependent on season or is it noon or night.
Renewable energy systems have various methods to store energy, which are divided into chemical, electrical, mechanical, and thermal storage solutions. For energy management, demand balancing, and moderate to long-term energy needs, suitable storage solutions are pumped-storage hydropower, compressed air energy storage, batteries, fuel cells, and thermal energy storages. For short-term energy needs and power quality maintenance, the best options are flywheels, batteries, and supercapacitor energy storage. Storage solutions are lacking large-scale energy storages, because pumped-storage hydroelectricity covers 99 % of the grid-connected electricity storage capacity. Thermal energy storages succeed as an off-grid solution, but other solutions must be developed to be more cost-efficient to operate in larger-scale as a grid-connected storage method.
Another key solution for intermittency is usage of the conventional energy sources as an adjusting power. Coal, petroleum, gas, and uranium can be used in a cycling plant, intended to meet the imbalance between the energy supply and demand. This approach is a quite straight-forward solution, but it causes the power plant to function with varying load, causing operational and physical difficulties for the system. The main adjusting properties that power plant needs are quick start-up, ability to increase or decrease power quickly, and a low minimum power output to maximize the adjusting range. Operation of the conventional energy generation systems has been changed from the baseload and peaking plants towards load-following and two-shifting. Flexibility procedures made for the power plants are even more increased when higher amount of renewable energy is integrated in the energy system.
Challenges of solar energy in Finland are low annual irradiation values, solar radiation concentration to summer months, and large locational differences within the country.
However, central Europe has succeeded in large-scale solar energy deployment with nearly the same annual irradiance values as Finland. The common belief that solar energy utilization in northern Europe is unprofitable has led to high market prices for solar panels and minimalistic financial support for solar energy technologies. In turn, for instance Germany has guided solar energy production prices to approach typical customer electricity prices by financial supporting. Finland has low adjusting potential for large-scale solar energy integration and energy storage systems do not function properly without right operating temperature. Still, the usage of consumer scale solar energy has potential by making PV-panels more affordable and by supporting their usage. Germany has proven over the last decade that solar energy growth comes from market-oriented thinking by offering PV-panels for low initial investment prices and allowing solar electricity pricing to follow typical electricity prices.
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APPENDICES
Appendix A. Intermittency Time-Scale Comparison Appendix B. Technical Specifications of Energy Storages
1. Batteries 2. Fuel Cells
3. Electrical, Mechanical and Thermal Energy Storages 4. Status and Typical Applications in Current Energy System
Appendix A. Intermittency Time-Scale Comparison
Table A.1 compares average energy production, predictability and cause of solar, wind and tidal energy and is based on Chapter 3, sections 3.1-3.3. Wave energy is not included as it is dependent on wind and its energy production globally is marginal. Table A.1 is divided into four time-scales: annual, seasonal, diurnal and hourly. The purpose of these time-scales is to show how intermittency affects to specific energy sources average energy production within that time-scale. Also, Table A.1 points out how predictable these variations are and explains the cause in a simplified form. By default, Table A.1 is intended for latitude 30 to 60 and is only rough overall estimation. It should be noted that intermittent energy sources are very dependent on geographical location.
As seen from Table A.1 solar energy is overall more predictable than wind energy and its average energy production fluctuates less within the time-scales. This average energy production still hides the true difficulties behind solar energy, because even though the fluctuations in average energy production are minor, it does not produce energy at night or in winter. Another problem for solar energy in high temperate latitudes is that demand spikes happen twice a day when it gets colder, at morning and at late evening, which are bad times for solar radiation. In the other hand, wind energy fluctuates more but usually produces energy all day without major stoppages. This means that wind energy can compensate the low energy production time periods of solar energy, while solar energy brings more predictability to the daily energy production pattern.
Table A.1: Average energy production, predictability and cause of solar, wind and tidal energy with annual, seasonal, diurnal and hourly time-scales.
Annual Seasonal Diurnal Hourly
Solar
Appendix B. Technical Specifications of Energy Storages 1. Batteries
Table B.1 accumulates technical specifications of the most commonly used batteries, focusing on presenting the operational specifications instead of theoretical ones. For example, the LA batteries may have 170 Wh/kg theoretical specific energy, but in practice they are in 25-50 Wh/kg range. The cycle life is based on DoD, which is considered by giving the best discharge level for every battery. In practice, one cycle life in Table B.1 means discharging battery to given DoD, not to DoD of 100 %. (Brett et al. 2006, 783;
Suberu et al. 2014, 503-506; Sudworth J. 2001, 152; Zhaoyin et al. 2008, 1698.) Table B.1: Technical specifications of most commonly used batteries for renewable energy sources (Brett et al. 2006, 783; Suberu et al. 2014, 503-506; Sudworth J. 2001, 152; Zhaoyin et al. 2008, 1698).
2. Fuel Cells
Table B.2 accumulates technical specifications of the most commonly used fuel cells.
FCs are still a small-scale technology with high-end construction materials, which has led to variable costs. The costs are still linked closely to qualified power. It is also good to keep in mind that the most used FCs use hydrogen (H2) as fuel. The DMFC uses methanol (CH3OH), DEFC uses ethanol (C2H6O) and MCFC coupled with SOFC uses reformate of carbon monoxide (CO) and hydrogen. SOFC has also option to use direct methane (CH4) as fuel. The specific energy of fuel cells is not listed, because FCs can be constructed very differently. However, the usual specific energy range of FCs is 800-10,000 Wh/kg, being the largest among all ESSs. (Suberu et al. 2014, 506-512.)
Table B.2: Technical specifications of most commonly used fuel cells (Suberu et al. 2014, 506).
Type η
(%)
Cost (€/kW)
Qualified power
(kW)
Operating temperature
(C)
Fuel
AFC 60-70 500-8000 10-100 70-100 H2
DEFC 20-30 500-8000 100-1000 90-120 C2H6O
DMFC 20-30 500-8000 100-1000 90-120 CH3OH
MCFC 50-60 500-8000 100-300 650-700 Reformate CO/H2
PAFC 40-55 500-8000 5-10,000 150-220 H2
PEMFC 30-50 500-8000 0.1-500 50-100 H2
SOFC 50-60 500-8000 0.5-100 800-1000 Reformate CO/H2
3. Electrical, Mechanical and Thermal Energy Storages
Table B.3 accumulates technical specifications of the most commonly used electrical, mechanical and thermal energy storage solutions for renewable energy sources. The cost is divided into two categories, €/kW and €/kWh, which tells if the ESS is economically viable as short-term or as long-term solution. The cycle life is only for the applications that have clear discharge process. Some of the devices also does not have specific energy or specific power, as they are dependent on other factors. For example, in PSH these depend on the height and the capacity of water reservoirs.
Table B.3: Technical specifications of the most commonly used electrical, mechanical and thermal energy storage solutions for renewable energy sources (Amrouche et al. 2016, 20921-20923; Demirel 2016, 324-343; Droste-Franke et al. 2012, 90-93; IEA 2014, 18-19; Kowal et al. 2011, 573; Luo & Wang 2013, 22-23; Tixador 2008, 5; Suberu et al. 2014, 501).
Type η
4. Status and Typical Applications in Current Energy System
Table B.4 summarizes status and typical application of different energy storage solutions in today’s energy system. Locations in the energy system are termed as generation, called supply, transmission and distribution, called T&D, and end-use, called demand. From applications arbitrage means storing low-priced energy and selling it during high-priced periods. The table has also some example projects for the technologies. The primary applications and locations may change in the future as the energy system evolves.
Table B.4: Status and typical locations for energy storage solutions in current energy system (IEA 2014, 17-20).
Type Location Output Primary
application Example project
SES T&D Electricity Short-term storage
Hybrid electric vehicles (R&D phase)
SMES T&D Electricity Short-term storage
D-SMES (United States)
CAES Supply Electricity Long-term storage, arbitrage
McIntosh (Alabama, United States) Huntorf (Germany)
FES T&D Electricity Short-term storage
Demand Thermal Low temperature applications
Shanghai Pudong International Airport (China)
Molten salt
Supply Thermal High temperature applications
Gemasolar CSP Plant (Spain)