Hydropower Site Assessment
& Design

A good hydro site is a valuable asset
You could write a book what makes a good hydro site, but in summary:

• Good head
• Good flow
• Simple site layout with main parts close together
• Good grid connection
• Good site access
• Single ownership of the site, or cooperative neighbours
• Not too many environmental sensitivities

Good head

In our hydropower projects, head is typically the most important factor in having a good hydro site, provided all of the other parameters are satisfactory. The power, and therefore energy output from a site is proportional to head. The cost of a hydro system is to a large degree determined by the physical size of the civil engineering structures and the turbine, so as heads get lower and water volumes increase (because the pressure and therefore velocities decrease), the system gets more expensive. The opposite is true as the head increases; the system gets physically smaller and costs less while at the same time the power and energy production increases, making the return on investment higher. This doesn’t mean that low head sites are bad – at the end of the day the head you have is what you have – but it does mean that all things been equal, higher head sites are better than lower head sites. It also means that for any site it is always worth doing whatever you can to increase the head and reduce head losses, which can all be done by good hydropower design.

Good flow

You can’t do anything without sufficient flow, so this is also important. Generally speaking, low head sites tend to have higher flow rates because they are in the lower-end of a river’s catchment in lowland areas, and therefore have a large catchment upstream with lots of tributaries. Higher head sites are usually higher in the catchment where the land is steeper (hence the higher heads possible), but also the catchment is smaller, so flow rates tend to be less.

This is no ‘right’ flow rate, it is just a matter of calculating how much energy you can generate and deciding whether it will be economically viable.

Simple site layout with main parts close together

This is mainly in relation to construction and installation works. The perfect site from a construction perspective is a green field, with no constraints. As complications arise such as historic structures, public rights of way, historic machinery and natural features (like cliffs, bed-rock, waterfalls, springs etc.) the project gets more complicated and therefore expensive. Very few projects are ‘perfect green field sites’, so don’t worry if you have some of the constraints listed above, but be aware that more constraints means more construction cost.

It is also beneficial if the intake, turbine location and discharge are all reasonably close together. On lower-head sites this ensures that the civil engineering (i.e. concrete) structures are relatively small, hence lower cost. On higher-head system it means the penstock-pipe is reasonably short, which is important because this is often the most expensive part of a high-head hydropower system.

Good grid connection

You may have a cracking hydropower site, but it is no good if you can’t sell the power because there is no grid connection! You not only need a physical grid connection, but it also needs to be ‘strong’ enough to take all of the power you will generate. Generally you will need a three-phase 11,000 volt supply either on-site or nearby. Ideally you would already have a suitable on-site transformer or substation so you can connect on the low-voltage (LV) side.

Very small systems (<25 kW) may be able to connect to a single-phase supply, and in some cases you can connect up to around 80 kW using a special ‘split-phase’ transformer.

It isn’t possible to tell how ‘strong’ a three-phase 11 kV supply is without formally asking the Distribution Network Operator (DNO). Renewables First can make grid connections applications on your behalf and obtain quotes for any grid upgrade works required. See here for more details.

Good site access

For the construction stage, and to a lesser degree for on-going maintenance, the site will need access for construction equipment, delivery vehicles, and for large systems, cranes. Hence narrow lanes, tight corners, soft ground and obstructions caused by buildings can all cause problems which in the worst case would prevent the project progressing, and in the best case may just mean additional cost. These issues would all be considered when the hydro feasibility study was carried out.

Single ownership of the site, or cooperative neighbours

In a perfect world the entire site would be owned by a single landowner. This not only makes the environmental consenting easier, but also means that commercial agreements between landowners are not required, which if they are usually means sharing a percentage the revenue. It is surprising how often such commercial agreements cannot be concluded because of irrational greed from one party, so it is well-worth discussing the project and how all stakeholders would be compensated at an early stage and getting the appropriate legal agreements signed before proceeding with the expensive consenting and design stages.

Not too many environmental sensitivities

As discussed in more detail on the consenting pages, the process of getting environmental consents from the Environment Agency in England and Wales is bureaucratic and slow, though it is slightly better under SEPA in Scotland.

In all regions the more ecologically-sensitive a site is, the longer and more expensive the consenting process will be. In general terms hydropower systems can be safely incorporated into all but the most sensitive environments, and will always be designed to have no negative impact, and will often provide a net improvement, particularly for fish passage.

So environmental sensitivities (like migratory fish, fish spawning, protected species etc.) are not show-stoppers, but do slow the consenting process down and make it more expensive.

The following hydropower project risks could all affect the viability of a hydropower project. Most of the risks can be significantly reduced by developing your project with an experienced hydropower designer / installer, such as Renewables First.

Consenting and outline design stage

• It is important to have written agreements in place if multiple-landowners are involved before any substantial work is completed.
• There is always a risk that the environmental regulator may not issue the required licences and consents, though this can be mitigated by good quality feasibility and pre-application work.
• Securing a grid connection can be a risk, particularly in remote locations, so permission to grid connect should be obtained at the outline design stage.
• Planning consent can be a risk, but is not normally an issue provided the system is well designed.

Detailed design/construction stage

• There is a risk that the ground conditions for foundations are not as good as anticipated, though this can be mitigated by reference to geotechnical survey data and on-site geotechnical analysis.
• Construction works are normally conducted during the drier parts of the year, but there is always a risk that severe wet weather will occur which can cause damage at the site and delay works.

Operational stage

• Regulatory licenses must be renewed every 12 years. There is a presumption of renewal, but if any new environmental legislation has come into force it is possible that the hydro system will need upgrading to meet the new requirements, at additional cost.
• The Feed-in Tariff is a Government scheme that could be subject to change, though recent legal challenges to changes have been won by the generators, which adds some confidence. The Feed-in tariff for new renewable energy projects was scrapped in April 2019.
• The annual energy prediction from the hydro system is based on long-term average flows, but year-to-year flows can vary significantly about this average: this can be very beneficial if you have a series of wetter years, but it can go the other way as well and have several dryer years.
• The export price for electricity can go up as well as down, though the general consensus is that energy prices will continue to rise at above-inflation rates for the foreseeable future.

Yes, but it is generally prohibitively expensive and not worth the extra complexity. Using hydro off-grid means that you need more expensive types of equipment / generator and much more complex control systems, and a means of storing energy (or dumping it) becuase the overall electrical system must be balanced.

In the UK we are very fortunate in having one of the best electricity distribution networks in the world. Most people only have one or two power cuts a year, and even these are often of short duration or during the night when it doesn’t really matter. Because our distribution network is so reliable, it is much simpler technically and much more cost effective to be grid connected, and effectively use the grid as a giant battery to dump surplus energy to and draw energy from as required – however if power to the grid fails the the hydro power system will shut down.

If you really want to use hydro off-grid then we can design and supply a suitable system, but as a ballpark estimate assume it will add 25% to the project cost, and the resulting system will be more complex and require more maintenance.

Yes, you can consume energy on site, provided that the wind turbine can connect into the site distribution board. Sometimes this is not possible because the wind turbine site is geographically separate from the site distribution board, or the cable run would be excessively long.

Whenever possible the preferred option would be to connect into the site distribution board because this allows you to offset imported energy which significantly increases the income from the wind turbine.

It is important to remember that electricity flows like water and will always follow the easiest route to the nearest load; this means that all of the site owner’s loads (i.e. lighting, sockets, machinery, air conditioners etc.) that connect to the same distribution board would be supplied firstly by the wind turbine, and only once all of these loads had been satisfied would any surplus energy from the wind turbine flow backwards through the incoming supply cables, either to the next nearest distribution board on the site, or out through the export meter to the grid.

Also, because the electricity produced by the wind turbine is fully grid-synchronised, it will mix seamlessly with grid-imported electricity. This means that if the wind turbine cannot supply all of the site’s loads, then all of the electricity from the wind turbine would go towards the loads and any deficit would be seamlessly imported from the grid.

Equally, if the wind turbine was supplying all of the local loads but then a reduction in the wind speed caused the output to suddenly drop, then the grid would instantly supply more to make up the deficit. From a consumers point of view the source of the electricity would be unknown; it could be from the wind turbine, the grid or a combination of both.

In the situation where the on-site loads far exceed what the wind turbine could ever produce, then all of the electricity generated by the wind turbine would be consumed on site. For example, if a wind turbine with a maximum power output of 500 kW was connected to a site that had a baseload (i.e. the minimum load 24/7) of 1 MW, then 100% of the energy generated by the wind turbine would be consumed on site.

Financially this would be a good arrangement because the price paid for importing electricity from the grid is typically 12 p/kWh (varies between 8 – 16 p/kWh depending on the import tariff), so if the amount of import could be reduced, for every kWh it was reduced by the site owner would save 12 p. If you compare this saving of 12 p/kWh to an export price of 6.5 p/kWh, you can see that offsetting on-site loads is worth almost double the value of exporting the electricity.

If it isn’t physically possible to connect to the onsite distribution board because the cable length would be too long, or the generation system is significantly larger than the onsite loads, or there simply isn’t an on-site distribution board, then the system would be directly connected to the grid via a dedicated grid connection.