These authors contributed equally to this work.

The current paper aims to develop and to apply a methodology for assessing the hydroenergy harvesting potential in water systems taking into account both technical and economic aspects. The methodology is a five-step procedure: (i) data collection and analysis; (ii) technology identification; (iii) energy harvesting assessment; (iv) economic analysis; and (v) final recommendation of the technological solution. The case study is located in the water source of the Alviela River, in Portugal. The energy harvesting potential is assessed for three turbine types, adequate for the 2.5 m available head: two propeller turbines, with and without adjustable blades, and the Archimedes screw turbine. Results show that the most feasible solution is the Archimedes screw turbine, with 3 m

Residential, commercial and industrial water consumers rely on water supply system (WSS) to transport, treat, and distribute water from sources to consumers, in the required amount and adequate pressure, in order to maintain their daily activities [

Many water and wastewater utilities have already made an effort to assess energy efficiency through the calculation of water energy balances [

Several hydro-turbine technologies can be used in MHP that can be divided into impulse turbines, which operate at atmospheric pressure (e.g., Pelton, Cross-flow or Archimedes screw turbines), and reaction turbines, which operate under pressure (e.g., Francis, propeller with fixed blades, Kaplan and Pump running as turbine). In addition, several unconventional solutions have been studied, such as: the use of tubular propeller turbine in pressurized systems [

The turbine selection must attend to the range of available heads and discharges [

WSS are complex systems in which the pressure head and discharge vary continuously and, for this reason, the chosen turbine operating range must fit these variations [

The current paper aims to develop and demonstrate a detailed technical-economic feasibility study for installing a MHP. It includes a description of the proposed methodology for assessing energy harvesting potential and the expected capital costs and net revenues for the analysed solutions. The methodology is applied to a real-life case study located in a 2.5 m height weir in Alviela river at the WSS source. The AST solution is compared with other technologies typically used for energy harvesting for small heads. Several economic indicators are calculated. The main conclusions are drawn concerning the use of this solution in systems with low heads. The key innovative features are: (i) the description of a comprehensive methodology for energy harvesting assessment applicable to any water system, taking into account data records of available head and discharge, as well as, the turbine discharge-head-efficiency curves, (ii) the demonstration of the methodology in a real case study for three technological solutions (i.e., AST, Kaplan and the Propeller), particularly appropriate for low-heads; and (iii) the discussion of the advantages and drawbacks of each technological solution.

A comprehensive methodology for assessing the potential of hydroenergy harvesting in water and wastewater systems and for selecting the most adequate energy recovery technology, taking into account the technical and economic aspects is presented herein. The methodology is a five-step procedure (

consists of identifying the primary purpose of the water supply or collectors system, its main characteristics and the potential locations for energy harvesting, which must combine excessive head, large discharges and sufficient available space to install the powerhouse and the turbine. For the identified locations, available head and discharge data must be collected and should include, at least, three consecutive years representative of the system operation. If necessary, it should be identified, the derived discharge that cannot be turbined. Above all, the construction of MHP cannot compromise the primary purpose of the system, which can be the water supply or the wastewater drainage or treatment.

focuses on the selection of possible technological solutions based on the available head and discharges range and the technical features of each solution. For this purpose,

consists of the simulation of the energy harvesting over one year for each energy recovery solution, considering the head and discharge data and different values of design discharge. The proposed method requires the following input data: the pressure head and discharge data over at least three consecutive years with a time-stamp that can vary between 15 min, in systems highly demand dependent (e.g., networks), and 1–5 days, in systems with high seasonal variation (e.g., transmission systems, storage tanks); the interval of acceptable design discharges; and the discharge range of operation of the turbine

Initially, the discharge probability of occurrence curve is calculated over the assessment period (e.g., three years) based on discharge time history,

The turbined discharge at simulated time step is determined considering the following rules: (i) if the available discharge,

The annual harvested energy,

The annual volume used by the turbine, the annual harvested energy and the total power are calculated for each design discharge.

consists of the economic analysis of the project based on the annual harvested energy for each design discharge. Capital costs, operation and maintenance (O&M) costs and gross and net revenues are calculated. Three economic indicators are typically used to evaluate the feasibility of the project: the net present value (NPV), the payback period (PBP) and the internal rate of return (IRR). These are calculated for each design discharge and for each selected turbine. The additional input data to calculate these indicators are: the discount rate,

consists of the comparison of analysed technological solutions to select the best technical and economical option for energy harvesting in the analysed water system. The main parameters to be compared for each design discharge scenario are harvested energy, CC, PBP, NPV and IRR. The final solution is the one that leads to the highest NPV with an acceptable IRR (>10%) and an adequate payback period ideally lower than 10 years [

It should be highlighted that the project design discharge depends not only on the energy harvested but also on the economic analysis, since higher discharges not only lead to higher revenues but also to higher capital and O&M costs.

A computational tool has been implemented in MATLAB for carrying out the described simulations.

Empresa Portuguesa de Águas Livres (EPAL), the water utility responsible for supplying Lisbon, is concern with the environment and aims to be increasingly a Zero-Energy consumer. It has identified the water source of the Alviela transmission system, as a potential location for energy harvesting with a high seasonal discharge and 2–3 m of available head.

The Alviela river source, located in Santarém district, Portugal, is currently managed by EPAL. The Alviela transmission system, built in 1880, represented a significant water source for Lisbon and other neighbouring municipalities 40 years ago. The system includes the Alviela transmission system, with over 100 km of length and a transport capacity of 0.8 m

During this study, a topographic survey of the Alviela source area was carried out (

The analysis of the discharge data series between 2004–2019, provided by EPAL, has shown that the period of 2016–2019 had an anomalous behaviour (

Four scenarios were considered for the water derived to the transmission system that could not be used for energy harvesting. The derived discharge is described as a percentage of the maximum capacity of the transmission system (

The probability of occurrence curve obtained for the total discharge and for the potentially turbined discharges (available discharge) in the period 2004–2015 are depicted in

The technological solutions adequate for the available head (

Based on turbiune manufacturer information, the AST turbine efficiency is high (80–85%) for a wide range of discharges (40–100%

The developed technical-economical model is run to estimate the harvested energy for the design discharges between 1.5 to 15 m

The annual harvested energy,

Economic analysis requires the calculation of the capital and O&M costs, gross and net revenues, as well as the economic indicators NPV, PBP, and IRR. Several assumptions are considered herein: (i) discount rate = 5%; (ii) project lifetime = 20 years; (iii) energy unit cost = 0.08753 €/kWh; (iv) annual O&M = defined as a percentage of the capital cost in MHP. The discount rate, project lifetime and unit energy cost are typical values used by EPAL. Different turbine manufacturers have been consulted to obtain the quotations for the electromechanics equipment (turbine-generator setup), for power ranges up to 150 kW. It was considered that the equipment corresponded to a percentage of the CC of the project [

In addition to the hydropower plant construction, it is also necessary to build a new 50-m pipe between reservoir R1 and the transmission system chamber for design discharges higher than 1.5 m

The obtained capital costs, O&M costs and gross revenues for the AST solution are presented in

The three technological solutions have been calculated for design discharges between 1 and 6 m

The best solution obtained for each technology is presented in

Despite having a lower efficiency, the AST has a lower unit capital cost (2 M€/MW) and also a lower annual O&M costs (0.5% of capital cost/year) than the other two propeller turbines. The increased harvested energy of Kaplan and Propeller turbines due to the higher efficiency (

Since the capital cost increases with the turbine power, the technical analysis results in higher discharges maximizing the harvested energy, while the economic analysis maximizes the net cash flows (revenues minus costs). Despite the produced energy increasing with the discharge until it reaches 12.0 m

Concerning the physical characteristics of the AST, based on manufacturer information of 3.0 m

This research work describes and demonstrates with a case study the application of a methodology for assessing the technical and economic feasibility of energy recovery in water systems. The recommended solution, the AST, has a rated power of 55 kW, a rated discharge of 3 m

Energy harvesting in the water industry is a growing area, that has raised interest of many recent research studies and real life projects. This is due to the existing energy harvesting potential and the need for making the industry more sustainable and energy-effcient. The most promising technology for the case study of Alviela is the Archimedes screw turbine, which is a relatively new technology, when operating as a turbine and not as a pump, never applied in Portugal.

The Águas de Portugal (AdP) Group, being EPAL one of its holdings, operates nationwide providing services to the municipalities, managing the multi-municipal systems and directly serving their populations for water supply and sanitation. The AdP Group has established a goal to achieve carbon neutrality by 2030 and has developed a ZERO Energy Neutrality Programme to reduce energy consumption and to boost the renewable energy production from biogas, wind, hydro and solar power. This programme includes the installation of hydropower plants with a total capacity of 6.9 MW with an estimated production of 45.0 GWh/year. Many of these hydropower plant locations have high discharges and low heads, like the presented case study. This paper compares traditional turbine solutions with the AST, highlighting the main differences and showing that the AST is a promising and cost effective technology for harvesting energy at locations with low heads (up to 10 m) and high discharges, such as, water river intakes and water and wastewater treatment facilities.

Conceptualization, P.F.G.O., N.M.C.M. and D.C.; methodology, P.F.G.O. and D.C.; software, P.F.G.O., N.M.C.M. and D.C.; validation, P.F.G.O., N.M.C.M., P.F. and D.C.; formal analysis, N.M.C.M. and D.C.; investigation, P.F.G.O., N.M.C.M., P.F. and D.C.; resources, P.F. and D.C.; data curation, N.M.C.M.; writing—original draft preparation, P.F.G.O.; writing—review and editing, P.F.G.O., N.M.C.M., P.F. and D.C.; visualization, P.F., N.M.C.M. and D.C.; supervision, N.M.C.M. and D.C.; project administration, D.C.; funding acquisition, D.C. All authors have read and agreed to the published version of the manuscript.

The authors acknowledge Fundação para a Ciência e Tecnologia (FCT) for funding the research Projects PTDC/ECI-EGC/32102/2017, iMIST—Improving Mixing in Storage Tanks for Safer Water Supply, and PTDC/HAR-HIS/28627/2017, HORTO AQUAM SALUTAREM—Water Wise in Gardens in the Early Modern Period.

Not applicable.

The data presented in this study are available on request from the corresponding author.

The authors declare no conflict of interest.

Proposed methodology for energy harvesting assessment and technology selection.

(

Alviela river source (

(

Results for the AST: (

AST economic analysis results as a function of the design discharge with and without the pipe cost: (

Economic analysis results as a function of the design discharge with and without the pipe cost: (

Parameters comparison between AST, Kaplan and Propeller turbines; (

AST schematic installation–top view.

Technical and economic parameters comparison of optimal solutions for each turbine.

Turbine | _{d}^{3}/s) |
Operation (h/year) | ^{3}) |
Pipe (k€) | CC (k€) | O&M (k€/year) | Net Revenue (k€/year) | NPV (k€) | PBP (years) | IRR (%) | ||
---|---|---|---|---|---|---|---|---|---|---|---|---|

AST | 3 | 55 | 282 | 5183 | 55.98 | 50.4 | 160.7 | 0.8 | 23.9 | 137.3 | 8.3 | 13.7 |

Kaplan | 1 | 22 | 160 | 7326 | 26.37 | 0 | 88.3 | 3.5 | 10.5 | 42.6 | 11.1 | 10.2 |

Propeller | 1.5 | 33 | 179 | 5479 | 29.58 | 0 | 99.3 | 2.0 | 13.7 | 71.7 | 9.13 | 12.5 |