Introducing flexibility in alpine small hydropower plants using smart storage

Small hydropower plants can improve their proficiency if they are able to become more flexible on power production, answering the need of peak energy demand, following the needs of the energy market and providing ancillary services in the electricity supply sector. Particularly during winter season, the discharge in alpine rivers is very low and the run-of-river systems, which do not have a reservoir, may need to switch off due to the lack of incoming water. During this period, the natural inflows may be higher than the environmental flow but lower than the minimum discharge for the turbines. During normal operations, these water volumes would be lost. The solution herein proposed and under test is therefore to store this water and to turbine it afterward. The storage of water is the most-used technique to increase the flexibility for the utilization of the water flowing down a river. On a run-of-river power plant, the first issue is to identify how a volume for water storage can be found. The options are either to build an open air volume, by means e.g. of a dam, or to use the underground volumes which are normally occupied by certain structures composing the scheme. Since the first option requires major investments due to the cost of construction, the use of the already existing structures, whose function will be revisited, is the most promising technique. In order to select the part of the system to be used as water storage, a series of considerations need to be taken into account: • The optimal volume for the water storage depends on the inflow and time frame we consider for the regulation (intra-day, intra-week or intra-month); • The energetic value of the water storage depends on its elevation with respect to the turbines; • The cost of the works needed to adapt the function of the component; • The possibility of safely change the use of the hydropower plant component i.e. which are the consequences and the limits for using those volumes to store water instead of keeping their intrinsic function. The analysis of the possible volumes to be used for storing water has been conducted on a systematic way looking first and foremost to the components of the power scheme, therefore avoiding costly operations of new excavations. A ranking for the most interest component of the scheme to be used as reservoir, can be constituted by analyzing the resulting energy coefficient and the use rate, which accounts for the number of times the reservoir is filled by water and how long it takes (a big reservoir may in fact be useless if the incoming water does not fill it completely and on a reasonable time). The hydropower plant can then be operated that as soon as the “reservoir” is filled, the turbines can use the water to produce energy at a constant discharge, which ideally is close to the optimal discharge for the given turbine, until the “reservoir” is empty. And those cycles may repeat many times during the day. The production of energy i.e. the use of t