Sizing Behind-the-Meter Solar PV Systems for Water Distribution Networks ^†

Abstract

This study investigates three methods for sizing behind-the-meter (BTM) solar PV systems for pumped water distribution networks (WDNs). The three methods are (1) the industry method based on current industry practices, (2) the minimum total life cycle cost (TLCC) method to minimize TLCC through the life of solar PV systems, and (3) the minimum payback method to minimize the time to pay off the capital investment in solar PV systems. The industry method risks over-sizing, while the minimum payback method risks under-sizing. The minimum TLCC method leads to systems with balanced performance. The findings offer decision-makers insights when selecting solar PV systems for WDNs.

1. Introduction

Water distribution networks (WDNs) are essential components of both urban and rural infrastructures. They deliver water from the source to various downstream users to supply the necessary volume, pressure, and quality for daily water use. The operation of these networks requires a substantial amount of energy due to pumping. As a pioneer in Australia, the Victorian water sector has committed to achieving net-zero emissions by the year 2035, in accordance with Victoria’s water strategy, known as Water for Victoria [1]. Therefore, to cater to growing water needs while also minimizing environmental impacts and economic costs, a number of water authorities have considered on-site renewable energy supply, such as solar photovoltaic (PV) systems. These are often referred to as behind-the-meter (BTM) solar PV systems [2]. The sizing of the BTM solar PV systems for WDNs is usually performed in an ad hoc manner and is dependent on the funding available. However, appropriate solar PV size can provide the integrated system with both economic and environmental benefits. Therefore, there is a need to establish a systematic way of determining the size of BTM solar PV systems for WDNs, taking into account the long-term performance of the combined water and solar systems [3].

In this study, we investigated three approaches for determining the size of BTM solar PV systems for WDNs. The first approach is an industry method based on the current industry practices for sizing solar PV systems in residential buildings (i.e., referred to as the industry method). The second is the minimum TLCC (total life cycle cost) method, which focuses on reducing the overall life cycle costs of the integrated system across the solar PV system’s standard service life of 25 years. The third method aims to minimize the payback period for the initial investment in the solar PV system, which is an important consideration in economic evaluations. A variety of performance indicators have been used to evaluate the performance of the integrated water–solar system, where the BTM solar PV system size was determined using these three methods. The three sizing methods are assessed through two case studies: one theoretical and one real-world WDN.

2. Methods

In this study, three different solar PV sizing methods have been investigated for WDNs. The first method, i.e., the industry method, determines a solar PV system size based on the most critical month in terms of meeting energy demand with solar energy generated in a typical year, and it seeks to maximize the utilization of solar energy generated during daylight hours. The second method, i.e., the minimum TLCC approach, determines the size of the solar PV system for a pre-existing water distribution network in a way that results in the lowest total life cycle costs for the combined water–solar system during the service life of the solar system. The standard 25-year service lifespan of a solar PV system is used. The third method, i.e., the minimum payback method, gives the size of the solar PV system in a way that minimizes the time it takes to pay off the initial solar PV capital cost. This method is based on the assumption that key stakeholders are interested in recovering their initial investment in solar PV systems as quickly as possible.

Determining the size of the solar PV system for a WDN using either the minimum TLCC or the minimum payback method can be formulated as a single-objective optimization problem. Solar PV sizes are taken as decision variables for the optimization problem using both methods, while the total life cycle cost and payback period are the objective functions that need to be minimized. The performance of the three solar PV sizing methods is assessed using three types of performance indicators, including (1) economic costs and benefits, (2) energy consumption, and (3) GHG emissions.

3. Case Studies

Two case studies have been investigated using the three sizing methods. The first one is a hypothetical network named ‘NET 1’, which consists of 8 demand nodes and 11 pipes. Water is pumped from the source to the eight downstream users. The second case study system is a real-world irrigation system—the Robinvale High-Pressure System (RVHPS) in Robinvale, Victoria, which is operated by the Lower Murray Urban and Rural Water Corporation (i.e., Lower Muray Water). In RVHPS, 244 irrigation users receive river-quality water that is directly pumped from the Murray River. Pumping demand is very high during peak seasons, leading to high energy consumption and associated GHG emissions.

4. Results

We take the economic performance results for the NET 1 system as an example. As shown in Figure 1a, the industry method results in the largest solar PV size of 537 kW, followed by the minimum TLCC method, which has an intermediate solar PV size of 345 kW. The minimum payback method leads to the smallest solar PV system of 105 kW. Consequently, the lowest TLCC of AUD 1.94 million is obtained from the minimum TLCC method, as expected. This is followed by the TLCC of AUD 2.02 million from the industry method and then the largest TLCC of AUD 2.27 million from the minimum payback method. Details of the breakdown of the total life cycle cost are shown in Figure 1b. The breakdown of energy consumption is shown in Figure 2a. A larger solar PV size can lead to less energy consumption imported from the centralized grid. However, a significant increase in the solar PV size from 345 kW (obtained from the minimum TLCC method) to 537 kW (obtained from the industry method) leads to a limited increase in the solar energy that has been utilized, potentially leading to a reduction in overall system efficiency. In addition, a larger solar PV size (e.g., 537 kW obtained from the industry method) leads to the lowest grid GHG emissions, as shown in Figure 2b. The minimum total life cycle GHG emissions (i.e., 10.1 kt) are obtained by using the industry method. Hence, the minimum total life cycle cost and GHG emissions cannot be achieved at the same time. The solar PV sizes obtained for RVHPS are shown in Figure 3a. Similar findings have been observed for RVHPS, as shown in Figure 3b and Figure 4a,b.

5. Conclusions

In this study, three different methods for sizing BTM solar PV systems for WDNs have been introduced and assessed against economic, energy, and emission performance indicators. The industry method yields the largest solar PV system, but it may potentially oversize the system. The minimum payback method results in the smallest solar PV system, but it may potentially undersize the system. The minimum TLCC method has achieved the minimum total life cycle cost and balanced performance. The rankings on all performance indicators can guide stakeholders in choosing appropriate solar PV systems. Decision-makers can select the method best suited to their specific requirements and priorities when choosing different performance indicators during their design process.