In the following graph:

What is the difference between "hydro" and "tidal/wave"?

And what is the difference between "solar (heat)" and "solar panels"?

  • Hi there, and welcome. Do you know the publication this originally came from? Was it the IEA 2004 World Energy Outlook, or something else?
    – 410 gone
    Nov 24, 2014 at 10:27

5 Answers 5


Beware that chart - it is from 2004, and hence is very out of date! In particular, photovoltaics ("solar panels") have become cheaper much faster than anybody expected a decade ago, and wave and tidal power have proved more difficult and costly than some had projected. Here's some more up to date info on costs: http://www.eia.gov/forecasts/aeo/electricity_generation.cfm


"Hydro" power conventionally refers to the use of a height difference between two bodies of water to generate electricity as water falls from one to the other. Typically this is achieved by damming a river, or running a pipe ("penstock") from a high lake to a lower lake, river or sea.


Tidal power refers to generating electricity from the flow of the tides. There are two approaches:

  • Exploiting the vertical rise and fall of the water level over a tidal cycle. A barrage is constructed with gates that close at high water, to retain the water inside the barrage while the water level outside goes down. Once a height difference has developed, the water inside is allowed to run out through turbines, operating much like the traditional hydro systems described above, but with a relatively small height difference. There are variations on this approach, but all exploit the cyclic change in water level in a similar way.

  • Exploiting the speed of a fast tidal flow. Some places in the world have exceptionally fast flows for various reasons, and this approach puts turbines in the water to extract energy from them, analogous to underwater wind farms.


Wave power aims to extract energy (usually electricity) from the periodic motions of swell waves - the large waves that you can see breaking offshore from a beach. There are a lot of different methods being developed as prototypes, and there is no clear leading approach as yet.


"Solar panels" on the chart probably refer to photovoltaic panels, which produce electricity from sunlight. This is what is commonly thought of when people refer to solar power.

"Solar (heat)" probably refers to the the use of rooftop panels that use sunlight to heat water for domestic use. However, the term could also be used to describe Concentrating Solar Power (CSP) plants, where sunlight is focussed on a large scale to heat steam (or another fluid), and that steam is used to drive a turbine to produce electricity. As noted in the comments, it could also refer to a few less common solar power techniques, such as solar updraught towers, but this is less likely as these technologies are poorly developed at present.

  • Solar (heat) could also refer to a [en.wikipedia.org/wiki/Solar_updraft_tower](thermal updraft tower) which uses the movement of hot air to spin a turbine. Nov 24, 2014 at 15:59
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    @DanNeely I guess it could, but has anybody got far enough with building one to give any idea as to the cost of energy? I'll edit in something.
    – Flyto
    Nov 24, 2014 at 16:10
  • A few test towers have been build according to Wikipedia. One that was operating at 200KW in 2008 was supposed to have been upgraded to 27.5MH by 2013. Unfortunately it appears that the upgrade was cancelled when proximity to an airport resulted in height restrictions. scmp.com/news/china/article/1487659/… Nov 24, 2014 at 16:53
  • Are all currents used for hydropower turbines tidal? I'm not sure. The big ocean currents for instance like the Gulf Stream are not tidal, although I'm not sure any ocean currents have ever been exploited.
    – Adam
    Oct 14, 2022 at 8:59
  • @Adam at the time of writing this, eight years ago, no ocean currents had been exploited. To the best of my knowledge this is still the case.
    – Flyto
    Oct 14, 2022 at 22:44

Hydro power uses dams (or at least a pipe where one end is as much higher as possible than the other) to use the pressure of water at the bottom to turn a turbine and generate electricity (or occasionally, do some other work such turn a mill). They are often located on the edge of a large lake in a mountainous area so that even when there hasn't been much rainfall recently there is still a good amount of water to use. For those interested, the power able to be generated is based on a simple formula which is: the flow multiplied multiplied by the "head" (the height difference between the surface of the water and the outlet where the turbine generates the power).

Tidal/wave power uses the flow of the tides or the back and forth motion of individual swells to generate electricity. These tend to be located in the narrow entrance/exit of a bay or other land form with a strong tidal flow (for tidal power), or in a location exposed to strong swells or waves (for wave power).

Solar refers to power from the sun, which can be collected as heat simply through heating of a collector (this is thermal/heat solar), or as electricity through photovoltaic solar panels. In general, examples include coiled black hose to warm water for a pool (solar thermal), silicon photovoltaic panels on the roof of a building to supply electricity, or even a parabolic mirror which concentrates sunlight into a small spot for providing a much more intense heat.

The graph provided specifically use the unit of dollars per kilowatt, so presumably is considering industrially sized electricity generation techniques. This means it won't be including the use of a black hose to heat your pool for solar thermal, but would include something like a solar tower with many mirrors that concentrate the heat enough to run a steam turbine.


There is some ambiguity in the table. If it's only about electricity generation, that helps a lot.


The two types of solar could get split in different ways. "Solar panels" usually refers to photovoltaic generation of electricity from semiconductors. Solar thermal could mean the directly supply of hot water from solar thermal panels or other thermal harvesting mechanisms. OR it could refer to what's also called concentrating solar power, CSP, whereby sunlight is concentrated - either by a huge array of tracking mirros onto a central tower, or by parabolic troughs onto pipes - and that heat is then used to drive a turbine to generate electricity. PV panels are nearing maturity now; CSP is still at the grid-scale commercial prototypes. There's low uncertainty on costs of PV, and on the rate of decline of PV costs - it's called Swanson's Law, and works similarly to Moore's Law for computers. The IEA have famously been rather inept on predicting how quickly deployment would occur, and how quickly costs would come down. There's high uncertainty on the costs of CSP.


Hydro refers either to storage hydro, where the water is held in an upper reservoir, and released through turbines embedded in a dam or in the flow channels downstream from it, to generate electricity; or to run-of-river hydro, where electricity is generated as the water flows down a river and is channeled through a turbine. Both are well-established technologies. There's very low uncertainty over costs of both run-of-river and storage hydro. (NB storage hydro is different to pumped-storage hydro; storage hydro is a source of primary generation; pumped-storage hydro is a means of storing surplus electricity as gravitational potential, and converting it back to electricity later, when needed.)


Wave power is a nascent technology, and harvests the kinetic energy of the waves through pneumatics or other mechanisms. There are a few prototypes in the water. There's very high uncertainty over costs of wave power.


Tidal stream is also a nascent technology, and uses the kinetic energy of the daily or twice-daily (depending on where you are in the world) to drive a turbine. There are some prototypes in the water, and there's very high uncertainty over costs of tidal stream.

Tidal barrages and lagoons use the height differences between high and low tides to drive water through turbines to generate electricity. Tidal barrages are established, and have medium uncertainty of costs. Tidal lagoons are a bit of a fad at the moment in some places, unproven, with high uncertainty on costs. This also applies to tidal reefs and other variants.

Tidal is distinct in one curious way from all the others above. All the others are solar power, either directly or indirectly. Tidal comes from gravitational energy, not solar energy. (wind is also indirect solar energy. Geothermal is different again, being partly a product of nuclear reactions in the crust, and partly the reservoir of heat from the creation of the Earth)


Hydro Power:Power generated via potential energy of water as it flows from a high to low area. This is the concept behind dams.

Tidal Power:Power generated by the movement of water according to tides. This is used in areas where the tide moves water in or out of a specific area, and that movement can be converted to electricity.

Solar(Photovoltaic):Power generated when light hits a panel containing semiconductors that create electricity.

Solar(Heat):Power generated when light is used to heat water and produce steam, which rotates a turbine to generate electricity. This is usually done using fields of mirrors that refl

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    Hi Andrew, welcome to Sustainability.SE! Your answer seems to have been truncated. You can edit it using the "edit" link below the answer.
    – Flyto
    Nov 24, 2014 at 17:34

i think that there is a difference between them as tidal energy is formed from the waves/tides and hydroelectric is formed from the height difference between two bodies of water.

  • Welcome to Sustainable Living! Can you please elaborate on your answer. As it is your answer doesn't provide anything over the answers we already have.
    – THelper
    Mar 24, 2017 at 8:02

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