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Analysis of energy efficiency solutions in water supply systemsAisha Mamade1, Dália Loureiro2, Dídia Covas1. Helena Alegre2

1 CERIS, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal2 National Civil Engineering Laboratory, Lisbon, PortugalKeywords: water-energy nexus, energy efficiency, cost-effectiveness

INTRODUCTION

Water supply systems are high energy consumers, with electricity costs varying from 5-30% of total operating costs. These costs affect the financial health of water utilities increasing tension over public budgets, which may lead to increasing water tariffs. Energy costs also represent the largest controllable operational expenditure of most water utilities and therefore, initiatives to improve energy efficiency (EE) in water supply systems have been increasing worldwide (Brandt et al. 2010, Park and Bennett 2010, Copeland 2014, Loureiro et al. 2016). According to the World Bank, commonly applied measures to improve EE generate 10-30 % energy savings and have 1- to 5-year payback periods. The main barriers of successful EE implementation originate in sector governance issues, knowledge gaps and financing hurdles. Efforts in data collection standardisation, training and capacity building will tackle down knowledge gaps (Liu et al. 2012).

The aim of this paper is to present energy efficiency solutions that derive from a capacity building project – iPerdas – whereby 24 water utilities received one-year training on water losses and energy efficiency. Three case-studies will be presented, showing a wide range of opportunities for energy efficiency improvements that goes from the typical solutions to others that are less commonly applied, but generate a good performance in terms of costs, GHG reductions, sustainability goals.

METHODOLOGY

The methodology followed in the current paper is integrated in the AWARE-P methodology for infrastructure asset management (Alegre et al. 2013) and focuses on improving the diagnosis of energy efficiency in water supply systems. As depicted in Figure 1, it can be applied in two levels: system and analysis areas.

Rev

iew

Objetives > assessmentcriteria > metrics > targets

Diagnosis

From level above

Produce Plan

Implement Plan

Monitor Plan

To lev el below

Data collection

1. Energy balance calculation

2. Performance assessment

3. Analysis of energy eff. alternatives

AWARE-P methodology

URBAN WATER SYSTEM

SYS

TEM

LE

VE

LA

NA

LYS

IS L

EV

EL

Figure 1 – Methodology for energy efficiency (EE) analysis

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The first step is to calculate an energy balance and assess the main components of energy consumption in the system. This requires collecting basic data such as inlet water volumes and hydraulic heads at delivery points, storage tanks and pumping stations and electric energy consumption in pumping stations. The second step is to calculate performance indicators that allow ranking the most critical. Performance indicators used in this paper are the following:

Ph5-Standardised energy consumption: physical IWA PI (Ph5) that refers to the average amount of energy consumed per volume of pumped water, standardized at a head of 100 m (Alegre et al. 2006).

E3-Ratio of total energy in excess: represents a ratio of the theoretic energy in excess that is supplied to the system in comparison to the minimum energy required areas (Mamade et al. 2015).

The third step is to identify problems and to propose solutions for each critical area and conduct a cost-effectiveness analysis for each solution. This included calculating the investments needs, savings and a simple payback period. Table 1 presents examples of EE issues, causes and solutions for improving energy efficiency.

Table 1 Examples of EE issues, causes and solutions for improving energy efficiency

Examples of EE issues Examples of causes Examples of solutions

High dissipated energy in pumping stations

Inefficient pumps (incorrect design, inefficient operation, old equipment, lack of maintenance)

Energy audits, pump/component rehabilitation, pump/component replacement, VSD installation

High dissipated energy in pipes

High friction losses (incorrect design, old pipes)

Pipe rehabilitation (e.g., change of diameter, material)

High dissipated energy in valves

High valve headlosses (e.g., pressure reducing valves (PRV), flow control valves)

Change valve regulation to avoid excessive energy dissipation, replace valves by microturbines

High surplus energy Inefficient system operation, Inefficient network layout

Establishment of operation rules that minimise energy costs (e.g., pump scheduling), re-designing network layout to reduce energy consumption

High energy associated with water losses

Real losses (e.g., pipe bursts, leakage and overflows in storage tanks); apparent losses (unauthorised consumption and metering inaccuracies)

Water balance calculation, Pressure management, rehabilitation, active leakage control, minimum night consumption analysis, replacement of old flow meters.

CASE-STUDIES

Case-study A-Pump scheduling

This case-study refers to a simple water supply and distribution system (Figure 2 (a)) with high water losses: only 54% of the total water supplied is consumed. Performance assessment results show that the pumping station has 46% efficiency (Ph5= 0.5 kWh/m3/100m) and that excess energy represents 3.3x the minimum energy (E3=3.3).

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0

1000

2000

3000

4000

5000

6000

7000

8000

Off-Peak 1 Off-Peak 2 Peak 1 Peak 2

Annu

al En

ergy

con

sum

ption

[kW

h]

Energy tariff

(a) (b)

Figure 2 – Water supply scheme (a) and annual energy consumption per tariff in the stato-quo (b)

Since the tank has a good storage capacity, there is an immediate potential for pump scheduling with no investment needed. Shifting energy consumption from peak to off-peak hours results in annual savings of 2700€. Due to high water losses, solutions to reduce both real and apparent losses will be explored in the near future. Further studies are needed to evaluate how water loss reduction will affect the pump setting point and, consequently, the pump´s efficiency.

Case-study B – Layout change 1

This case-study refers to a rural water supply. The main issues identified include an inefficient pumping station and high pressures in some areas associated with the topography. Figure 3 presents the network model with pressures at 3:00 am.

Tank

Pump

Flow direction

Tank

Flow direction

(a) (b)

Figure 3 – Results for pressure with stato-quo (a) and alternative layout (b).

The diagnosis included the calculation of a simplified energy balance and energy performance indicators. Diagnosis results showed that Ph5=1.44, meaning that pump efficiency is 19% and E3= 4.5, meaning that excess energy is almost 5x the minimum required energy. Studied solutions include: (i) re-designing the pump to improve its efficiency; (ii) replacing flow meters installed for more than 12 years, and (iii) re-designing the network layout by connecting the distribution system to the conveyance system and de-activating the storage tank and the pumping station. Alternative (iii) proved to be the most effective in the long-term, since. it completely reduced energy consumption

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(Ph5=NA) and allowed more adequate serviced pressure as presented in Figure 3 (b). This results in a decrease of the excess energy to 1.9x the minumum energy.

Case-study C-Layout change 2

This case-study refers to a transmission main with 25 km that is supplying water to a tank 75 m below the upstream (Figure 4). This tank has a good storage capacity to ensure summer water demand, but is not necessarily needed in the low season. A pressure reducing valve is installed in the transmission main to minimise excessive pressures. Water is then pumped to another tank, using a pump that has low efficiency (Ƞ=20%). This path has an E3=1.4, meaning that excess energy represents 1.4x the minimum energy. An alternative layout where water goes directly to the distribution area without using the pumping station during the low season would cut the excess energy in half (E3=0.7). The possibility of replacing the PRV by a PAT to recover energy should also be explored.

50

100

0

Alternative layout ( ):Replace PRV by PAT and build alternative layout so that the pumping station is used during peak season only.

E3=1.4PRV

L=25 km

Ƞ=20%

E3=0.8

E3=0.7PAT

L=25 km

Ƞ=20%

(a) (b)

Figure 4 – Results for E3 in stato-quo (a) and alternative layout (b)

SUMMARY OF RESULTS

Table 3 summarises the obtained results.

Table 2 Examples of EE issues, causes and solutions for improving energy efficiency

Case-study

Current solution Alternative solution

Ph5 E3 Ph5 E3 Energy consumption reduction Investment

kWh/(m3.100m) - kWh/(m3.100m) - kWh/year €/year €

A – Pump scheduling 0.5 3.

3 0.5 3.3 0 2700 0

B – Layout change 1 1.4 4.

5 - 0.9 4422 530 24750

C – Layout change 2 1.5 1.

4 1.5 0.7 22368 3579 Still being

assessed

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FINAL REMARKS

This research work analyses energy efficiency solutions in three water supply systems. Different energy efficiency issues have been raised and different solutions have been drawn. Some solutions are commonly applied and focus on equipment efficiency improvement (e.g., Pump scheduling), while others requires a system approach that identifies system design improvements (e.g., re-designing network layout), water loss reduction (i.e., less water needs to be pumped) and energy recovery (e.g., replacing PRV by microturbines). Improving energy efficiency is pivotal to reduce energy costs and consequently, reduce operational costs and meet sustainability goals.(Alegre et al. 2006, Mamade et al. 2017)

References

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Alegre, H., S. T. Coelho, D. Covas, M. C. Almeida, and A. Cardoso. 2013. A utility-tailored methodology for integrated asset management of urban water infrastructure. Water Science & Technology: Water Supply 13.

Brandt, M. J., R. A. Middleton, and S. Wang. 2010. Energy efficiency in the water industry: a compendium of best practices and case studies - Global Report. 10/CL/11/3, Global Water Research Coalition.

Copeland, C. 2014. Energy-Water Nexus: The Water Sector’s Energy Use. Congressional Research Service.Liu, F., A. Ouedraogo, S. Manghee, and A. Danilenko. 2012. A primer on energy efficiency for municipal water and wastewater

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Inefficiencies: Results from a Real Water Supply System. Procedia Engineering 119:1098-1108.Park, L. and B. Bennett. 2010. Embedded Energy in Water Studies-Study 2: Water Agency and Function Component Study and

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