Storage is a hot topic across the energy industry these days and hydropower has an important role to play. Storage of water using dams can very significantly improve the capacity factor and therefore generation of an operational scheme.
Modelling and feasibility
The operation and therefore benefit of a storage reservoir needs to be calculated using time-series flow data for the site. This is not often available or there is not enough data for it to be of meaningful use. Therefore we have developed our own tool to enable time-series flow data to be synthesized from the more commonly available frequency-domain flow data (i.e. a flow duration curve). This allows us to conduct storage modelling on any site, regardless of the availability of flow data.
The other fundamental influence on reservoir storage is topography, particularly for relatively low dams. Detailed topographic or bathymetric surveys of the reservoir area can be essential to determine the potential storage volume and how this varies with water level. For large new impoundments we often use a combination of drone and terrestrial survey methods to gather the data at minimum cost.
Reservoir storage can be used to directly feed the hydro penstock or to attenuate the flow of water upstream of a screened intake. Storage connected to the hydro penstock is always controlled by the turbine control system, it is effectively an integral part of the hydro scheme. Attenuation storage can be applied when a new scheme is being designed but is most commonly used as a ‘retrofit’ to an existing scheme. Attenuation storage may be actively controlled using a valve, or be passive, where the discharge is regulated by the size of the outlet pipe.
When modelling attenuation storage the aim is usually to optimise the modified flow regime downstream of the reservoir to suit the performance of the hydro scheme. However, in some cases it is also important to consider the behaviour of the reservoir itself, particularly the range and rate of change of the water level. This is particularly true where the reservoir is in a sensitive area (see below).
In some scenarios it is possible to repurpose or adopt existing reservoirs, commonly ex-water supply dams. These bring with them their own challenges in terms of condition assessment and paperwork. We have experience of working with Scottish Water to re-purpose redundant dams to feed a hydro penstock including the modelling and condition assessment of the structure.
Consents and outline design
Obtaining consents for storage dams is similar in nature to consenting a hydropower scheme but it typically requires additional flood analysis and should take account of good dam design practice to ensure that the consented structure will be acceptable to the panel engineer when it comes to detailed design. For this reason we often engage with a panel engineer at a relatively early stage in the project to ensure that the approach that is being taken is acceptable to them.
In sensitive areas it is often important to provide detailed information on the ‘level regime’ – the range of water levels and the rate of change. This may be to mitigate concerns over landscape impact, to mitigate the risk of flooding ground-nesting birds during the breeding season or to ensure that the ecology in the shallow margins is acceptable.
We have extensive experience of conducting detailed assessments of water levels including ongoing collaboration with ecologists to inform the modelling. This includes sites within Special Protection Areas, SSSIs and Special Areas of Conservation. There is often a trade-off between a storage regime that is of most benefit to the hydro versus the environment. In sensitive sites it can be important to demonstrate a net environmental gain, this can significantly improve the likelihood of obtaining consent.
In common with our approach to consents more generally, we strongly believe in early and detailed consultation with all interested parties.
Detailed design and implementation
We normally conduct the detailed design of dams in house, in accordance with best practice. We work with a number of panel engineers and engage with them from the outset of the design process. For larger structures or those that are more complex or have challenging ground conditions, we work with a range of structural and geotechnical consultants.
Dam construction requires particularly careful planning and oversight. The role of the panel engineer needs to be understood by all parties and it is particularly important to ensure that the contractor understands their authority and the importance of complying with the design and their requirements.
The control regime that is applied to a reservoir has a direct influence on the benefit. With a poor control regime the reservoir may offer no benefit at all. It is surprisingly common that the control is not optimised. We use a combination of data from the reservoir and our own reservoir models to compare performance and model the effect of different control regimes. Where appropriate this can lead to the modification of the control system for the dam (e.g. a turbine or valve control strategy), the adjustment of valve/orifice settings on passive systems and potentially the conversion from passive to active control.