— The True Role of Pumped-Storage and Why Next-Generation Geothermal & Renewables Should Lead the Future


—

Lead-in | “Nuclear Alley,” Mountain Batteries, and What Lies Beyond

If you live in the Kansai region, the landscape of energy can feel strangely extreme.

Quiet fishing villages and rice fields along the Sea of Japan. Just beyond the hills lies “Nuclear Alley,” home to the Takahama and Mihama nuclear power plants.
Further inland, deep in the mountains of Hyogo Prefecture, you find the Okutataragi pumped-storage power plant opening its mouth inside the rock.
And up in the Northern Alps, the Kurobe River No. 4 Power Plant—Kuroyon—draws power from the iconic Kurobe Dam.

> “A device that quietly produces heat inside a nuclear reactor”
vs.
“A giant water battery carved into a mountain”



Both of these very different things are supporting the same Kansai power grid.

As someone who became severely disabled in midlife and whose own body’s energy management is fragile, I’ve started to look at this energy landscape with a much more personal sense of urgency.

In this article, I want to dig into questions like:

How much power do the Takahama and Mihama nuclear plants actually produce?

Compared to that, what roles do Okutataragi, Okochi, and Kuroyon play?

Going forward, should nuclear remain the main actor, or is it time for next-generation geothermal and renewables to take the lead?


We’ll compare the Takahama Nuclear Power Plant, the Mihama Nuclear Power Plant, the Okutataragi and Okochi pumped-storage plants, and Kuroyon from the perspectives of power output and efficiency, and from there:

Clarify the true position of pumped-storage, and

Explore a future in which next-generation geothermal and renewable energy are the main players


from both a technical and a lived-experience point of view—as a mid-career, severely disabled blogger watching all this from a rural corner of Tajima.


—

What You’ll Learn (Summary)

Let’s start with the key points.

The Takahama and Mihama nuclear plants play the role of large-scale “base-load” power sources that produce huge amounts of electricity for long periods.

The Okutataragi and Okochi pumped-storage plants are giant batteries that store excess electricity as water and release it when needed.

Kuroyon produces less power than a nuclear unit, but is crucial as a hydropower hub that integrates the entire river basin.

Pumped-storage is not just a device to soak up excess nuclear power—
it’s on track to become core grid infrastructure that supports all renewables, including solar, wind, and geothermal.

Japan has world-class geothermal potential but has developed only a small fraction of it.

If next-generation geothermal (EGS, closed-loop, supercritical geothermal, etc.) is commercialized, we can realistically build a power system in which
“geothermal + hydropower + pumped-storage” × “solar + wind + batteries” forms a multi-layered backbone.

In such a system, pumped-storage will no longer be just a “minor hydro plant,” but will evolve into the beating heart of a renewable-centric power grid.


With that map in mind, let’s walk through it step by step.


—

Table of Contents

1. Basic Concepts: Nuclear vs Hydropower vs Pumped-Storage


2. Output Scale & Role of the Takahama & Mihama Nuclear Plants


3. Output Scale & Features of Okutataragi, Okochi & Kuroyon


4. Role Sharing Seen Through Efficiency: Nuclear vs Hydro vs Pumped-Storage


5. From “Nuclear + Pumped-Storage” to “Renewables + Pumped-Storage”


6. Japan’s Changing Power Mix and Policy Direction


7. What Is Next-Generation Geothermal? EGS, Closed-Loop, Supercritical


8. A Future Where “Pumped-Storage + Next-Gen Geothermal + Renewables” Lead


9. What Tajima & Northern Hyogo Reveal About the Role of Rural Regions


10. Conclusion: From “Who’s the Star?” to “How Does the Team Work?”


11. FAQ




—

<a id=”sec1″></a>

1 | Basic Concepts: Nuclear vs Hydropower vs Pumped-Storage

Before comparing anything, let’s quickly line up the basic ideas.

1-1 Nuclear Power: Basics and Key Traits

Nuclear power uses uranium fuel to produce heat through nuclear fission,
then turns that heat into steam, which spins turbines to generate power. Mechanically, it’s “a very complex steam engine.”

Key traits:

Huge amount of energy per kilogram of fuel

Very good at running continuously at a steady output (base-load power)

Bad at frequent output adjustments or stop-start operation

High-level radioactive waste and severe accident risk

Very long construction and decommissioning time, and high cost


Think of it as a big, constant-output power source: once it’s on, you really want to keep it running steadily to get your money’s worth.

1-2 Dam Hydropower: Basics and Traits

Dam hydropower uses the gravitational potential energy of water at height:

Water at high elevation

Downward flow

Turbine rotation

Electricity


Key traits:

Fuel is just “water,” so almost no direct CO₂ emissions

Turbines + generators are very efficient—often around 90%

By managing reservoir levels, you can adjust output to some extent

Once built, plants can last for many decades—some over 100 years

Large dams bring serious environmental and social impacts


Plants like Kuroyon are not just power plants—they also play roles in flood control, water supply, and tourism.

1-3 Pumped-Storage: Basics and Traits

Pumped-storage is “the advanced version” of dam hydropower.

1. When there is excess electricity, you pump water from a lower reservoir to an upper reservoir.


2. When electricity is in short supply, you release water from the upper reservoir and generate power.



In other words, it’s a giant water battery.

Key traits:

Converts electricity → gravitational potential (water at height) → electricity

Because there are losses in both directions, round-trip efficiency is about 70% (i.e., about 30% loss)

In return, it can store and release huge amounts of energy over hours to tens of hours

Ramp-up is very fast: it can often reach full output in a few to tens of minutes

It’s less a “power plant” and more grid-level balancing infrastructure


So pumped-storage is best understood as “equipment for editing the time distribution of electricity”, rather than as a simple generator.


—

<a id=”sec2″></a>

2 | Output Scale & Role of the Takahama & Mihama Nuclear Plants

Now let’s look at two key nuclear plants: Takahama and Mihama.

2-1 Takahama Nuclear Power Plant: A Major Pillar of Kansai’s Base-Load

Takahama, in Fukui Prefecture, is one of Kansai Electric’s core nuclear plants:

Unit 1: 826 MW

Unit 2: 826 MW

Unit 3: 870 MW

Unit 4: 870 MW


Total: 3,392 MW

If all four units were to run at full output for 24 hours:

> 3,392 MW × 24 h ≈ 81.4 GWh per day



If we assume a typical household uses about 10 kWh per day, this is:

About 8.14 million households’ daily consumption


Of course, actual operation changes with demand, inspections, and refueling.
But it’s clear that Takahama is a massive base-load power source for the entire Kansai region.

2-2 Mihama Nuclear Power Plant: A Compact Veteran

Mihama was originally a three-unit plant with a total of 1,666 MW,
but today the main unit in operation is:

Unit 3: 826 MW


Roughly the same as a single unit at Takahama.

So, in regional terms, Mihama is not small at all,
while in system-wide terms it’s a medium-size base-load plant.

Combined, Takahama and Mihama have:

> About 4,218 MW of nuclear capacity.



The big question going forward is:

> How do we support, back up, and ultimately replace this base-load capacity?




—

<a id=”sec3″></a>

3 | Output Scale & Features of Okutataragi, Okochi & Kuroyon

Next, let’s look at the hydro side.

3-1 Okutataragi: One of Japan’s Largest Pumped-Storage Plants

The Okutataragi Power Plant, in Asago City, Hyogo Prefecture, is one of the largest pumped-storage plants in Japan.

Type: Pure pumped-storage

Upper reservoir: Kurokawa Dam

Lower reservoir: Tataragi Dam


Effective head: About 385 m

Max output: 1,932 MW


If it runs at full power for 8 hours:

> 1,932 MW × 8 h ≈ 15.5 GWh



How big is that?

Compare it with Takahama’s full-day output of about 81.4 GWh:

Okutataragi’s 8-hour burst is roughly one-fifth plus of that.


So Okutataragi is:

> “A power dam that can absorb or inject an amount of power comparable to several nuclear units, concentrated in just a few hours.”



3-2 Okochi: Pumped-Storage with Nuclear-Class Output

The Okochi Power Plant, in Kamikawa Town, Hyogo, is also a pumped-storage plant operated by Kansai Electric.

Max output: 1,280 MW

Four turbines, each 320 MW


This 1,280 MW is:

About 1.5 times Mihama Unit 3 (826 MW)

Roughly “one nuclear unit plus alpha”


So one pumped-storage plant can, in principle,
inject or absorb more power than a single nuclear unit—and do so very quickly.

3-3 Kuroyon: Symbol of Hydropower and Basin Control

The Kurobe River No. 4 Power Plant (Kuroyon), using water from the Kurobe Dam, has:

Max output: 337 MW


In pure numbers:

Smaller than a Takahama unit (826–870 MW)

Smaller than Okutataragi or Okochi


But Kuroyon’s real significance lies elsewhere:

It’s the core of a hydro plant cascade across the Kurobe basin

It combines flood control, water supply, and power generation

It’s a symbol of Japan’s post-war reconstruction and high economic growth


So Kuroyon is more than a number.
It’s a symbolic and functional node in both energy and land management.


—

<a id=”sec4″></a>

4 | Role Sharing Seen Through Efficiency: Nuclear vs Hydro vs Pumped-Storage

Now, let’s compare these technologies through the lens of efficiency.

4-1 Nuclear: High Fuel Efficiency, Low Thermal Efficiency

Nuclear power has extremely high fuel efficiency—the energy per kilogram of fuel is enormous.
But the thermal efficiency (heat → electricity) is usually only around 30–35%:

About 65–70% of the heat becomes waste heat released into the sea or atmosphere

That’s why plants need high capacity utilization to be economical


The fact that nuclear plants are hard to stop and not designed for frequent ramping is structurally linked to this thermal cycle.

4-2 Dam Hydropower: High Efficiency Thanks to Simple Mechanics

In the case of dam hydropower:

“Water at height” → “turbine + generator rotation” → “electricity”

The mechanics are relatively simple, so efficiency around 90% is common.


Of course, this assumes you have water in the reservoir—

Precipitation and snowmelt patterns matter

Climate change also affects inflows


So there are constraints, but from a conversion standpoint, hydro is extremely efficient.

4-3 Pumped-Storage: 70% Round-Trip, But a Huge System-Level Benefit

For pumped-storage:

It converts electricity → pumped water → electricity

So round-trip efficiency is only about 70%—about 30% is lost.


At first glance:

> “If we lose 30%, isn’t that a waste?”



That’s true if you only look at one plant.

But at the power system level, the question is:

If we throw away excess power (forced curtailment), isn’t that worse than 30% loss?

If we don’t have any buffer, won’t we be forced to use expensive, CO₂-heavy peaking plants more often?


So pumped-storage is fundamentally:

> Not just “an inefficient battery,” but a buffer that reduces system-wide loss and risk.



It absorbs surplus when power is cheap or abundant, and
releases power when the system is tight.

That’s where its true value lies.


—

<a id=”sec5″></a>

5 | From “Nuclear + Pumped-Storage” to “Renewables + Pumped-Storage”

By now, we have a picture:

Takahama & Mihama: large, stable base-load plants

Okutataragi & Okochi: huge power batteries that modulate that base load

Kuroyon: a hybrid node of hydropower and basin control


5-1 The Showa–Heisei Model: Nighttime Nuclear Surplus → Pumped-Storage

In the Showa and early Heisei eras, Japan’s power system looked like this:

Daytime: high demand → nuclear + thermal + hydro run hard

Nighttime: demand drops, but nuclear is hard to turn off


So the idea was:

> Use nighttime surplus (nuclear + thermal) to pump water up,
then use that water to cover daytime peaks.



This was the classic “nuclear + pumped-storage” combo.

If you look at planning documents from the era when Okutataragi and Okochi were built,
you’ll see the intent: “Make use of nighttime nuclear surplus.”

5-2 The Reiwa Model: Flattening Renewable Variability with Pumped-Storage

But in the 2020s, the situation is changing.

Variable renewables—especially solar and wind—are growing fast.

Some regions already see midday solar overgeneration and output curtailment.

The nuclear share is much lower than in the past.


In this context, pumped-storage is shifting from:

> “An accessory for nuclear surplus”



to:

> “Storage infrastructure that supports all renewables—solar, wind, geothermal, etc.”



In practical terms, it means:

Use pumped-storage to absorb midday solar surplus in spring and autumn

Discharge in late afternoon and evening, when solar drops and demand rises

Use pumped-storage to smooth out sudden output changes from wind


So, in the Reiwa era, pumped-storage is starting to play the role of a common backbone for the entire renewable fleet.


—

<a id=”sec6″></a>

6 | Japan’s Changing Power Mix and Policy Direction

Let’s zoom out and look at Japan’s overall power mix.

6-1 Current Situation: Renewables at ~20–30%, Nuclear in Single Digits

Recent data for Japan shows:

Renewables (including hydropower): roughly 20–30%

Nuclear: single-digit to around 10%

The rest: fossil fuels (LNG, coal, oil)


Compared to the era when nuclear provided 30%+, this is a big shift.

Renewables include hydro, solar, wind, biomass, etc.

Solar, in particular, is growing, covering a big share of daytime load.

As a result, the importance of “balancing & flexibility” is skyrocketing.


6-2 Targets for 2030–2040: 40–50% Renewables, ~20% Nuclear

Japan’s energy policy now aims for:

40–50% of electricity from renewables by around 2030–2040

About 20% from nuclear

The remainder from high-efficiency thermal plants + hydrogen/ammonia, etc.


The big question is:

> What will the composition of that “40–50% renewables” actually look like?



If it’s mostly solar, the day–night gap becomes extreme.

Offshore wind is promising but needs grid expansion and coordination with fisheries.

Biomass faces fuel supply constraints and CO₂ accounting debates.

Large new dams are difficult; hydropower growth will mostly come from upgrading existing facilities.


This is why I pay close attention to next-generation geothermal.


—

<a id=”sec7″></a>

7 | What Is Next-Generation Geothermal?

— EGS, Closed-Loop, and Supercritical Geothermal

Next-generation geothermal is essentially “Geothermal 2.0.”

7-1 Japan: A Geothermal Giant That Has Barely Started Drilling

Japan, sitting on the Ring of Fire, has:

Geothermal resource potential on the order of tens of gigawatts

Often ranked third in the world in potential


But actual installed capacity is only in the hundreds of megawatts.
So we’re using just a few percent of what we could.

Reasons include:

Conflicts and negotiations with hot spring operators

Restrictions on development in national parks

High exploration and development costs

Long lead times before plants become operational


7-2 Three Unique Strengths of Geothermal

Even so, geothermal has three unique advantages:

1. Stable Output (Base-Load)

Unlike solar or wind, geothermal can theoretically run 24/7/365.

It can provide a base-load similar to nuclear or some thermal plants.



2. Domestic Energy

The “fuel” is heat beneath Japan’s own land, so import risk is low.

This is huge for energy security.



3. A Renewable That Can Also Support System Stability

Properly designed, geothermal output can be somewhat controllable,
allowing it to provide flexibility in addition to energy.




That is, geothermal can potentially combine the good parts of nuclear (steady output)
with the good parts of renewables (domestic, low carbon).

7-3 The Three Keywords of Next-Gen Geothermal

Next-generation geothermal can be broadly grouped into three types:

(1) EGS — Enhanced Geothermal System

Injects water into low-permeability hot rock to create artificial fractures and circulate water.

Could enable use of a much wider area beyond conventional high-permeability reservoirs.

Requires careful handling of induced seismicity and environmental issues.


(2) Closed-Loop Geothermal

Installs closed pipes underground; a fluid circulates inside the pipes to pick up heat.

Because the fluid is separated from groundwater, it’s easier to co-exist with hot springs.

Can potentially develop geothermal near existing hot spring areas with lower impact.


(3) Supercritical Geothermal

Taps ultra-high-temperature fluids (400°C and above) at deep levels.

Very high energy density but technically challenging.

If commercialized, could deliver large output from few, very high-capacity plants.


Most of these are still in research or pilot phases,
but roadmaps typically envision demonstrations in the 2030s and early commercial deployments by the 2040s.


—

<a id=”sec8″></a>

8 | A Future Where “Pumped-Storage + Next-Gen Geothermal + Renewables” Lead

Let’s imagine a near-future Japanese power mix.

8-1 Viewing the Power Mix as a “Team”

Imagine 2040 with:

Renewables: 40–50%

Nuclear: 20%

The rest: thermal + hydrogen/ammonia, etc.


One desirable configuration might be:

> “Geothermal + hydro + pumped-storage” form the backbone for base supply and balancing,
and on top of that we layer “solar + wind + batteries.”



In that framework:

Geothermal is the steady foundation.

Hydro manages river basins and mid-term balancing.

Pumped-storage covers short-to-mid-term balancing (hours to tens of hours) as a giant grid battery.

Solar & wind provide cheap, abundant primary energy.

Batteries handle shorter-term, fine-grained balancing (minutes to hours).


It’s a multi-layered, cooperative team rather than a single hero.

8-2 Using Takahama, Mihama, Okutataragi & Okochi as an Example

Recall the numbers:

Takahama (4 units): 3,392 MW

Mihama Unit 3: 826 MW

Okutataragi (pumped-storage): 1,932 MW

Okochi (pumped-storage): 1,280 MW

Kuroyon (hydro): 337 MW


Put them all on one map:

Base-load: Takahama, Mihama, and future geothermal

Balancing/Storage: Okutataragi, Okochi, plus batteries

Stable hydro + basin management: Kuroyon, other dams

Variable generation: solar & wind


Together they form a single integrated “team” in the Kansai and surrounding regions.

In that team, pumped-storage is:

> A hub that can absorb surplus from any source—geothermal, nuclear, thermal, solar, wind—and release it when needed.



8-3 Reframing the Question: Not “Nuclear vs Renewables,” but “How Much, How Long, for What Role?”

Once you see the system as a team,
the question becomes less about:

> “Nuclear or renewables? Which side are you on?”



and more about:

> “How much nuclear, for how long, and for which roles?”
“How quickly can we build up geothermal + renewables + storage to soft-land away from heavy nuclear dependency?”



Personally, I think a realistic, lower-risk path is:

Short-to-mid term: Use existing nuclear plants with strict safety measures, while aggressively investing in renewables.

Mid-to-long term: As geothermal, hydro upgrades, pumped-storage, solar, wind, and batteries expand,
gradually reduce nuclear dependency in planned steps.


A kind of quiet, controlled soft landing, rather than a sudden swing.


—

<a id=”sec9″></a>

9 | What Tajima & Northern Hyogo Reveal About the Role of Rural Regions

Now I’d like to bring in a more local perspective.

I live in Tajima, in northern Hyogo Prefecture, as a mid-career, severely disabled blogger.
In my daily living area, there are:

The Okutataragi pumped-storage plant (Asago City)

The Okochi pumped-storage plant (Kamikawa Town)

Various small hydropower and dams

Numerous hot spring areas (symbols of geothermal