Mohammad Eskandari - Hydro-Electric Power Plant

Hydro-Electric Power Plant Design (Torrente Grigno)

In this Project we comprehensively designed a Hydro-Electric Power plant in the Torrente Gringo which is one of the many rivers present in the Trento province, in the north east of Italy. The whole region has always exploited hydropower sources in order to compose their electricity mix with the biggest fraction possible of hydropower. This new project is a run-of-river plant, composed of an intake weir, a forebay (surge) tank, a penstock, a power house with a vertical axis Pelton turbine coupled to a generator and a tailrace/draft tube. The water of the Torrente Grigno is diverted (in a variable percentage according to the river flow rate in that give moment) by the intake weir, channeled to the forebay tank and then enters the penstock. This pressurized duct delivers the water to the five nozzles feeding the Pelton turbine, which moves the generator. Water then rejoins Torrente Grigno via a draft duct.

The project has to overcome a series of very strict environmental, economic and regulatory limits. First, we have to analyze the very complex hydraulic scheme of Torrente Grigno: this new pant should not damage the productivity of any already existing plant by law, so the intake and tailrace locations are not flexible.

Main Technical data

The following table sums up the main technical features of the project presented, complying with the Italian national laws as well as with the one of the autonomous Trento Province:

Preliminary economical evaluation

In order to do a reasonable feasibility study the presented business plan by Costabrunella.srl is going to be evaluated, and then we will propose our own one. We begin to approach their cost splitting and notice that most of the total costs are related to the penstock: for this type of plants it’s quite normal having a strong incidence of civil facilities and of course the long, high pressurized duct is the most expensive of them all.

Proceeding with their cost analysis we got form to the company their funding plan: the whole initial investment will be covered by a fixed 4% rate mortgage lasting 20 years.

We calculated all the economic indicators, such as: LCOE, npv, Payback time, total interest, capital splitted by year, O & M, and etc.

To conclude their business plan review, we could say that, as often happens with renewable plants, the incidence of the incentives is important: this is a strong uncertainty as nobody can really predict them in the future.

Legal context and permitting procedure

Studying the regulatory framework which concerned the plant was extremely complex due to the plant location: Trentino Alto Adige is an autonomous region, thus it has many laws of its own as well as the faculty to decide not to apply some national ones. All the required environmental verifications concerning constrains on emissions, noise, historical and archeological areas, hydrogeological (all the area is under “vincolo idrogeologico), flora and fauna were performed. Great care was place in particular in minimizing the impact of the plant over the carps’ reproductive cycle as requested by the bespoke ichthyological commissioned by the local population.

Validation and computation of the river flow rate data

The data of the river flow rate we later used for calculations were measured by Costabrunella.srl on Torrente Grigno every quarter of hour for 3 years, which is the most accurate way to get a reliable measure the average flow rate. However, we tried to independently check the data. By comparing the behavior over the months of the three year we have found a consistent trend if we neglect the winter months

As this empirical method has a relatively wide error margin, we also tried another approach: verifying consistency between the Grigno flow rate and the amount of rain fallen in the province (which were provided to us by ISTAT, the Italian national institute of statistics). In particular the trend of the rain and the flow rate must be same between mid-summer and the beginning of winter, as while in winter snow falls and remains in the glaciers (so the flow rate of the river remains low), during spring and up to June the snow melting effect sums and trumps the one of rain falling. This trend is always observed over the three years, with a perfect coupling in the most important autumn months (note data have been scaled):

To properly use the data we have to compute the power producible: firstly we need to sort the data from higher value to lower one. This approach should be done for the three years and then make an average. But before this we should consider the DMV (Deflusso Minimo Vitale) due to regulations.

After subtracting these amounts (DMV) from the flowrate of the river we will get closer to the flowrate which is going inside the turbine. But there is another consideration related to the maximum possible flowrate which is going inside the turbine that is 1 m3/sec except for the three months of July, August and September which is 0.75 me/sec. to allow kayak activities on the river. Thus, we filter the data from above to be in our favorable condition. Finally, we should sort all these data from higher one to less because we want to calculate the average of these data for the three year. The result has been shown in this figure:

From the three plots we can see a limited dispersion and a relative uniform behavior, being this a river of glacial origin. We can draw from this the total flow duration diagram on which we based our energy production estimation:

Technical analysis

The idea is trying to understand if the configuration of a single Pelton turbine with five nozzles chosen the owner of the plant is the most effective from both a technical and economical point of view. With the aid of the software “HPP design” we began by looking for the optimal machine for our data of height and flow rate:

By considering the plot that we showed above we analyzed different configurations

One vertical axis Pelton
One vertical axis Francis
Two vertical axis Pelton turbines in parallel

The project as it was proposed by Costabrunella.srl is focused on a single, vertical axis, Pelton 5 jets design: we began by considering this layout, with the ultimate goal of estimating the total energy production. All of our calculations were based on an average value of the flow rate of Torrente Grigno computed over the data available for the last three years.

We considered other configurations of the turbines and we compared the amount of produced energy to find best solution for our project.

However considering the nominal power as the maximum electrical power given to us by hpp-design the equivalent hours turn out to be a little higher than 5000 for the single pelton configuration, about 4800 for the francis configuration and about 5080 for the parallel configuration with the pelton turbines. In the end it’s important to consider comparing these data with the productivity’s prevision by Costabrunella.srl: they provide a provisional data of 6352 MWh yearly, and then we got quite near them.

Sensitivity analysis: Turbine choice

In this final section we are going to try to analyze various possible scenarios for the energy market, in order to understand whether the plant will be profitable over the next years and at which conditions. The business plan of “Costabrunella s.r.l.” company considered the current feed in tariff: to amplify its impact on the energy produced we applied the current tariff applied for 30 years (to try to evaluate all the three configurated plants on a more stable condition). On the payback time we have considered unacceptable any tariff value resulting in a time longer than 30 years.

Under the assumption of a costant feed in tariff for 30 years the LCOE are computed. These values therefore are the the selling price where NPV=0 and are 0,072 €/kWh for the francis turbine, 0,068 for the 2 pelton configuration and 0,065 for the configuration of 1 pelton.

Of course, strong feed in tariffs can push the payback time below ten years, favoring in particular the single Pelton design. The IRR reaches acceptable values above 8% for a feed in of 0,08 €/kWh for the single Pelton case, to higher values for the others; seemingly, the single Pelton design is also the one capable of guaranteeing the best NPV.

Sensitivity analysis to flow rate

Being this a relatively small hydro plant, we decided to further investigate the behaviour of the IRR, NPV and payback time if we considered a lower flow rate available

We also did much more sensitive analysis by variation of Penstock diameter, Effects of compensation Operas, Adaption of PPA. That you can find in our Report below.

PS:

Our project score was 29 out of 30, and it was second top ranked projects of course.
More information are available upon request
click HERE for downloading the Report.