| 摘要 | ABSTRACT
1. Background and Issues
One of the obstacles to promoting the use of renewable energy in the power generation sector is that variable renewable energy (VRE) such as solar PV and wind power may undermine the reliability of the power grid. In fact, a higher proportion of VRE to entire power generation sources is likely to make it difficult to maintain constant balance between the supply and demand of electric power on a real time basis.
Meanwhile, there are various technologies to help maintain the reliability of the power grid by supplementing VRE. For instance, Energy Storage System (ESS) stores excess electricity for laster use during a power shortage or eases power output fluctuation caused by VRE so that the reliability of the power grid can be enhanced with frequency and voltage maintained within a certain range.
However, the general view is that ESS costs ��much more�� than flexible facilities such as the operational reserve of coal-firing and gas turbines. There has been little research on the comparative analysis of economics among such technologies in relation to the reliability of the power grid. More research should be done on this subject in order to achieve the target ratio of renewable energy to total power mix in a cost-effective manner, while at the same time ensuring the reliability of the power grid.
This research analyzed the enhancement effect of the reliability of the power grid by ESS, with a focus on ��frequency and voltage fluctuations�� caused by VRE. In addition, the cost of reliability enhancement was calculated in the frame of economics analysis so as to compare with that of coal-firing and gas turbine. Implications were drawn from the analysis result, which should be considered in the dissemination of VRE such as solar PV and wind power in the power generation sector for the enhancement of the reliability of the power grid.
2. Research Results
The Korean government plans to increase the proportion of VRE to total power generation to 7.5% by 2035. Power generation by VRE is possible only intermittently depending on weather and climate, such as sun, wind and cloud, and therefore the power output of VRE is highly variable. This poses as a constraint to promoting the use of VRE for the power grid, given the existing power generation system, technologies and operation environment.
According to the 6th Electricity Demand Supply Basic Plan of Korea, 17 GW of wind power are planned for construction by 2027. Once the new wind power system is connected to the grid, according to Cheon Young-hwan (2014), it may cause problems with the reliability of the power grid because the maximum power output of wind power may sometimes decrease more than the size of the operational reserve. Such fluctuations can be deteriorated when solar PV is added to the power grid, which was not included in the analysis of Cheon Young-hwan(2014), which necessitates countermeasures to ensure the reliability of the power grid.
The result of simulations conducted in this research shows that BESS (lithium ion battery) can cope with the power output fluctuation of VRE and effectively stabilizes the frequency and voltage of the power grid. In case the power output of a 1MW solar PV system is fluctuated by 5%, 10% and 15% each, and is supplemented by BESS with the capacity of 50kW, 100kW and 150kW, they could control the fluctuation by 4.8%, 9% and 13% respectively, which amounts to 96%, 90% and 87% of the BESS capacity. In addition, voltage fluctuation associated with power output fluctuation was reduced by 70%, 65% and 65%, respectively.
In case the power output of a 2MW solar PV facilities is fluctuated by 5%, 10% and 15% each, and is supplemented by BESS with the capacity of 100kW, 200kW and 300kW, the fluctuation of 4.75%, 9.5%, and 14.25% could be controled by BESS, respectively, which amounts to 95%, 95% and 95% of the their capacity. Also, voltage fluctuation decreased by 70%, 63% and 66%.
According to the economics analysis result of this research, BESS is a less costly solution to supplement the power output fluctuation of VRE than coal-firing and gas turbine, and it is even more so when used to supplement wind power. The LCOE of a 20MW solar PV facilities is 242.3 Won/kWh. When a 1MW BESS (5% of the power system capacity) which supplements 4.8% of fluctuated power output is installed, LCOE increases to 251.9 Won/kWh. The difference, 9.6 Won/kWh, is the cost to supplement 4.8% power output with the BESS. In case 1MW BESS (5% of the power system capacity) which supplements 4.75% of fluctuating power output is installed to a 20MW wind power facilities, LCOE increases by 6.7 Won/kWh compared with operating without BESS. If BESS is not used, coal-firing or highly flexible gas turbine should be used to make up for lost power output, and their LCOE is 62.3 Won/kWh and 119.6 Won/kWh each. A simple cost comparison shows that the cost of BESS is lower than coal-firing and gas turbine for both solar PV and wind power systems. Even so, BESS is more cost-effective when used for wind power than for solar PV.
According to the result of economics analysis on BESS cost for household solar PV systems, the use of BESS proved economical for part of households which consume much electricity and pay big utility bills, and for those homes on islets, solar PV+ESS system was less costly than supplying electric power from the grid. In 2014, the LCOE of residential solar PV+ESS system was 267.3 Won/kWh, which was 35% higher than 198 Won/kWh of solar PV without ESS. ESS proved to be an economical solution for households which consume more than 501kWh/month, and there were 335,000 households which fall into this category as of 2013. Also, the cost of solar PV + ESS system was less costly for residents on islets such as Ulreung-do, Dukjeok-go, Wi-do and Chuja-do, than supplying from the grid.
Assuming that the installation costs of residential solar PV and ESS system decrease by 20% and 30% each by 2017, LCOE will fall to 206.9 Won/kWh. In this case, solar PV and ESS system can become an economic solution to households which spend more than 401kWh/month. There are 1,182,000 households which fall in this category as of 2013, accounting for approx. 7% of all 16,782,000 households. 206.9 Won/kWh is equivalent to 83% of power supply cost in Jeju island, 249 Won/kWh, as of the end of 2013. If electricity tariff from the grid is rationalized, 206.9 Won/kWh will become an affordable level of cost for households to adopt residential solar PV and ESS system even without government subsidies.
3. Policy Suggestions
A. Solar PV and wind power facilities of utility level
The research finding shows that a greater proportion of solar PV and wind power to the entire power mix may undermine the reliability of the power grid in Korea. If Korea is to achieve the 7.5% target ratio of VRE to total power generation by 2035 as planned, the government should explore various means to ensure the reliability of the national power grid and set up a comprehensive plan which should be thoroughly implemented. Through this research, a set of implications were found as below, which need to be considered for the expansion of ESS use as a supplementary means to promote grid reliability along the dissemination of VRE.
First, ESS provides benefit to the power grid by easing the power output fluctuation of VRE. However, because the developers of VRE cannot internalize such external benefit while ESS adds to cost, they have little incentive to use ESS. This phenomenon is what is called a market failure caused by external effects and therefore the government's intervention may be needed in order to let VRE developers internalize the external benefit provided by ESS. In other words, the part of power generation cost equivalent to the amount of external benefit should be compensated for in the form of government subsidies, etc.
Second, even if the solar PV or wind power developers internalize the external benefit of ESS, they may not still be able to recover all cost associated with ESS. The best way to reduce the gap between actual ESS cost and recovered cost is to induce the fall of ESS prices. Thus far, the effects and wider use of renewable energy technologies have increased the economies of scale and helped to push down the cost of using such technologies. In that context, more aggressive R&D support should be provided to boost the development of renewable energy technologies, in addition to government subsidies to encourage the use of ESS as aforementioned.
Third, the government has recently adopted a policy to encourage the use and investment of ESS within the framework of renewable energy support policy, which is to include ESS in RPS so as to give a weight of REC. In order to ensure the policy reaps intended results, support for ESS per unit should be greater than that of existing renewable energy facilities.
That's because BESS adds to cost but actual power production volume decreases due to charge, discharge and stand-by losses, which brings up production cost, which compares to existing renewable energy whose production volume goes up in proportion to investment cost.
Fourth, it should be considered to differentiate the amount of support for each type of renewable energy source which uses BESS. In case a BESS (10%) is used, the gap between incremental investment cost increase and incremental production cost increase was different between solar PV and wind power, 1.53 times and 1.46 times each. While the gap seems to be not so significant, the total production cost of solar PV increased by 19.7 Won/kWh, and wind power increased by 13.5 Won/kWh, and the gap expanded as the size of BESS gets bigger.
Lastly, more information gathering and additional research should be done on technologies in detail so as to decide what should be the reasonable level of support. For instance, charge, discharge and stand-by loss of BESS has rather substantial impact on production cost of power, and lack of information to prove the impact makes it difficult to decide what the reasonable level of support should be.
B. Implications for residential solar PV facilities
For some of electricity-guzzling households, solar PV+ESS already reached grid parity. If ESS installation cost continues to decline, such households will be willingly adopt ESS even at their own cost, without government support. While no official statistics are available yet, people in the power generation industry figure there is a fast increase in households which adopt solar PV systems even without government subsidies. Nevertheless, high upfront cost of investment is still an obstacle to the further dissemination of ESS, say, among consumers who are around the grid parity level. More efforts should be made to foster an environment to facilitate the use of ESS.
First, the reform of the energy pricing system will help facilitate the use of solar PV in households even without government subsidies. If utility tariff charged to households and residents of islets is raised to a level matching power production cost, the economics of solar PV+ESS will be further improved and more households will reach grid parity.
Second, the government should implement measures, as a short-term policy, to promote the use of solar PV+ESS by households who reached grid parity. To that end, various business models should be developed, tailored to different income levels of households.
Third, the government should foster an environment where households which reached grid parity (thus, the economics of solar PV + ESS is secured) are motivated to actively use ESS even without subsidies. One of such measures, for example, is to disqualify those which reached grid parity from government-subsidized schemes so that they are induced to embrace solar PV system. And this measure is already in place by the government.
Fourth, eco-friendly culture should be formulated across society through policy measures to promote public consent and non-economic incentives, so as to encourage the use of solar PV system. For instance, institutions with high public credibility may publish reliable data and information for public education and help consumers to make informed decisions. That way, overall public confidence in solar PV can be improved.
Fifth, incentives should be offered to households which have conditions advantageous to solar PV installation, such as sizable land, so as to encourage them to install large-capacity system. Under the current regulations, solar PV of 10kW or higher capacity is excluded from net-metering. Therefore, the disposal of surplus power is quite limited and there is little incentive to install large-capacity system. The revision of such regulations, thus increasing the minimum installed capacity eligible for net-metering, will enhance the economics of solar PV and encourage the use of large-capacity systems. |
