Present status of pumped hydro storage operations to mitigate renewable energy fluctuations in Japan

1 Introduction

Since the Great East Japan Earthquake followed by the nuclear accident at Fukushima Daiichi power plant in 2011,many limits of the electricity system,which has been established 60 years ago,have been revealed.Vertical regional monopolies,inflexible operations of cross-regional interconnectors,limited competition in the retail market are examples of these.Then electricity system reformation started in 2013 (still ongoing) based on the Expert Committee Report and the Cabinet Approve to realize a stable supply.One of the principles of the reforms is to use market mechanism in wider areas beyond former monopolized regional areas.To realize it,the following three items have been set and conducted:(1) Establishment of the Organization for Cross-regional Coordination of Transmission Operators (OCCTO),(2) Full competition in retail and generation sectors,(3) Unbundling of the transmission and distribution sectors.OCCTO started in 2015.Balancing market is now being designed by this organization and the Ministry of Economy,Trade and Industries (METI),and they aim to start it from 2021.

At the same time,feed-in-tariff scheme has been introduced for the installation of renewable energy since 2012.The purchase price of solar photovoltaic (PV) was set as 40 JPY/kWh at first,then rapid installation started,especially of solar PV power plants.Fig.1 shows purchased record of renewable energy under FIT scheme,it is found that the amount is rapidly increasing,and four-third is occupied by power generation from solar PV.Concomitantly,operation modes of PHES plants are gradually changing.OCCTO established control rule to meet power supply for power demand when a shortage of downward balancing power would occur as a part of unbundling of the transmission and distribution sectors [2].According to this rule,PHES plants are operated as top priority plants in downward balancing power.Therefore,PHES plants have started to mitigate fluctuations of variable renewable energy (VRE:solar PV and wind),especially in areas where renewable energy has been significantly installed.

Fig.1 Actual purchased amount of renewable energy installation under FIT scheme [1]

According to Shafiqur Rehman et al.[3],this situation that PHES plants operate along solar PV,is technically simulated and also demonstrated in preceding studies [4-13].Many of those studies focused on remote areas or islands,but current situation occurred in Japan as PHES plants and solar PV,is being conducted on the area of over 15 million habitants at least.Furthermore,all PHES plants were basically built for stable operations of base load power plants such as nuclear power plants at night time in Japan.Power demand was created by PHES plants at night and PHES plants supplied power for peak demand in the day in the past.However,operating mode of PHES plants started to change due to rapid VRE installations as shown above in some areas.Hence,it could be said that present status as PHES plants’ operations in Japan is a mixture state between the past operation mode to create demand for base load power plants and the recent operation mode to mitigate fluctuations of VRE.

This paper focuses on current operating situation of PHES plants,and transition from the past operations to the recent operations as PHES plants are discussed in order to clarify issues for transition across Japan.Historical development of PHES plants and current situation of VRE in each area are described in Chapter 2.PHES operations are analyzed from the viewpoint of capacity factor in Chapter 3 and then issues are identified.Finally,Chapter 4 shows conclusions.

2 Pumped hydro energy storage (PHES) plants

2.1 Dispatch rule for power plants

In Japan,the PHES is operated under the dispatch rule by OCCTO.The order is shown in Fig.2.The PHES is operated as the 1st order at the same order of thermal power control on line.The PHES is not operated in the market and the generation power company shall be borne not only the loss of power from pumping and generating,but also the grid tariff for the loss of power.

Fig.2 Dispatch rule for power plants

2.2 Existing PHES plants in Japan

The historical trend for the installed capacity for PHES plants [14]and nuclear power plants [15]are shown in Fig.3.The number of PHES plants increased together with the installation for nuclear power plants,especially for the period from the 1980s to 1990s.PHES plants are located in midway between nuclear power plants and demand areas,such as Imaichi PHES plant located between Fukushima (nuclear power plants) and Tokyo (demand area).

2.3 Comparison of installed capacity with variable renewable and nuclear

The comparison of installed capacity with variable renewable energy and nuclear [16]is shown in Fig.4.It is found that the installed capacities of VRE and nuclear are almost the same in each area.

Fig.3 Installed capacity of pumped hydro and nuclear in Japan 1970-2014

Fig.4 Installed capacity for variable renewable (VRE) and nuclear in each area in Japan

3 Analysis of operating conditions of PHES plants

As described in Chapter 2,after grasping the situation of the PHES plants in Japan,this chapter uses the supply and demand data [17]of each electric power company during the year 2018 from January to December.Using the ratio of the amount of pumped hydro power generation to the installed capacity of PHES plant as an index “pumped hydro ratio” (this is a kind of capacity factor),the amount of electricity generated by VRE,which is VRE to the total supply (excluding pumped hydro and cross-regional interconnector) as an index “VRE ratio”,the relationship between these two indexes are analyzed.

In this study,we will focus on Kyushu,Kansai,and Chubu areas three of the nine balancing areas in mainland Japan.This is because Kyushu area has a high VRE penetration and nuclear power plants have been re-started.In Kansai area,the VRE ratio is not very high but nuclear power plants have been re-started.Lastly,in Chubu area nuclear power plants have not been re-started,but it is an area where demand is large.

The installed capacity of pumped hydro power plants in Kyushu area is 2.3 gigawatts (GW),in Kansai area 5.2 GW,and in Chubu area 4.7 GW.The input of pumped hydro power was taken as the value which divided the output by 0.92.

In Section 3.1,the relationship per hour (equivalent to GW) is analyzed,and in section 3.2,the relationship per day (equivalent to gigawatt hours (GWh)) is analyzed.

3.1 Hourly basis for pumped hydro ratio depending on VRE ratio in each area

Fig.5 Hourly basis on pumped hydro ratio and VRE ratio in Kyushu area

Fig.6 Hourly basis on pumped hydro ratio and VRE ratio in Kansai area

Fig.7 Hourly basis on pumped hydro ratio and VRE ratio in Chubu area

In this section the relationship between VRE ratio and pumped hydro ratio are analyzed on an hourly basis focusing on the Kyushu.Kansai,and Chubu areas.The results are shown in Fig.5,6 and 7.From this,it can be seen that in Kyushu area,the ratio of VRE increases and the pumping operations of pumped hydro are actively performed.On the other hand,in Kansai area,the VRE ratio is 30% or less,which is about half that of the Kyushu area.Furthermore,even if the VRE ratio increases,the pumped hydro ratio does not increase at the same rate.However,in most cases where the VRE ratio is close to zero,the pumped hydro ratio is about 50%,and it is presumed that the combined operation with conventional nuclear power plants is still performed.Moreover,in Chubu area,even if the VRE ratio might be about 50%,it is found that the pumped hydro ratio rarely exceeded 50%,that is,the PHES plants are not used so much.

3.2 Daily basis for pumped hydro ratio depending on VRE ratio in each area

Whereas the previous section is on an analysis per hour,this section performs an analysis per day.The conditions are the same as in the previous section.With regard to the operation rate of pumped hydro,both the pump up and power generation are regarded as the same operation,taking absolute values.In this section,the focus is on the same areas as in section 3.1.The results are shown in Fig.8,9 and 10.

Fig.8 Daily basis on pumped hydro ratio and VRE ratio in Kyushu area

Fig.9 Daily basis on pumped hydro ratio and VRE ratio in Kansai area

Fig.10 Daily basis on pumped hydro ratio and VRE ratio in Chubu area

In Kyushu area,it can be understood that pumped hydro is operated steadily as the VRE ratio increases.On the other hand,in Kansai area,the VRE ratio is low and there is no trend in the operation rate of pumped hydro according to that ratio.In Chubu area,regardless of the VRE ratio,the operation rate of pumped hydro rarely exceeds 10%,and even when viewed per day.It is found that the PHES plants are not used as much as per hour.

3.3 Supply and demand balance on typical date in each area

Based on the analysis results of the previous two sections,the balance of supply and demand on typical day in each area is analyzed.It is also focused on Kyushu area,Kansai area and Chubu area.The day is selected when the operation rate of pumped hydro per day is the highest in each area.In other words,Kyushu area is on 3rd May 2018,Kansai area is on 18th July 2018,and Chubu area is on 18th February 2018.The figures are shown in Fig.11,12 and 13.

Fig.11 Demand and supply balance in Kyushu area on 3rd May 2018

Fig.12 Demand and supply balance in Kansai area on 18th July 2018

Fig.13 Demand and supply balance in Chubu area on 18th February 2018

From this analysis,in Kyushu area,pump up operation of pumped hydro takes place during the day almost mirroring electricity generation from solar PV,and electricity is generated from pumped hydro power plants during the time zones when the electricity generation from solar PV is small or zero in the morning,evening,and at night.It is very efficient operations to compensate for the fluctuations.

In Kansai area,unlike operations in Kyushu area,it can be seen that pumping up of pumped hydro is performed at night when demand is low,and power generation is being performed during the daytime.This may be due to the conventional operations of keeping the output of nuclear power generation constant and absorbing fluctuations by pumped storage power generation although the ratio of VRE may be small compared to Kyushu area.

Finally,in Chubu area,similar to Kyushu area,it can be seen that pumped hydro is pumped up during the day when the power generation of photovoltaic power generation increases,but its operation rate is only around 10%,it seems that there is still room to consider for more efficient operations.

4 Conclusion

The operations of PHES plants in Japan has changed from adjustments for the nuclear power plants to adjustments for variable renewable energy such as solar PV and wind power.

In Kyushu area,the transition of PHES plants’ operations has already started with solar PV under current dispatch rule of OCCTO.

Reference