The Water–Energy Nexus
A water-energy nexus analysis of a public university in California
By Angela R. Ruberto, Juneseok Lee, and Adam Bayer
In this study, water and energy use data (2006–2011) from water wells are analyzed for San Jose State University (SJSU). SJSU is a four-year public university with an enrollment of 30,000 and located in San Jose, CA. It is found out that water and energy use correlate each other, and their use decreased since 2008, due to SJSU sustainability movement. Water savings have significant impacts on associated energy savings, and they double the benefits in economic and natural resource savings aspects. The research outcomes will be disseminated to public relations within the University to promote sustainability movement within SJSU.
National Academy of Engineering (2008) announced 14 major challenges that will have to be addressed in the 21st century—many of the challenges are related to sustainability of water and energy. The Leadership in Energy and Environmental Design (LEED) program is also encouraging sustainable use of water and energy (UGBC 2006). Nobel Laureate Richard Smalley emphasized that energy and water will be the top problems of humanity for the next 50 years. How we use energy and water will be the key engineering approaches for the future. In most developed countries, centralized grid systems constitute the major energy and water supply infrastructure. When these infrastructures were founded, the energy and water were not expensive and people did not envision that these resources would be unsustainable.
Recently, ASCE assigned a grade of D- to water infrastructures within the US (ASCE 2009). Leakage rates (water lost while transporting water from treatment plant to tap water) within drinking water infrastructures reaches 32% in some utilities. Pump stations use significant portions of energy (up to 90% of the energy cost expended in water supply and treatment systems) to provide appropriate pressure for delivering treated drinking water through pipe network systems. It is clear that water leakages are consuming energy resources quite significantly. About 6 billion gallons of fresh water is known to be lost every year (AWWA 2007). In this vein, reduced demand for potable water could also translate to energy savings and associated carbon footprint reductions. The two resources are closely linked, as water is required to produce energy and energy is required to treat and transport water. Water-related energy use accounts for about 19% of California’s electricity, 30% of its natural gas, and 88 billion gallons of diesel fuel every year (Krebs 2007).
By 2025, more than 2.8 billion people in 48 countries will face either water stress or water scarcity conditions (“Solutions” 1998). It is expected that water scarcity will result in a shortage of freshwater that limits food production, deteriorates the environment or ecosystems, and prevents economic development. Water is required to produce energy, and energy is required to use water appropriately, which are termed “water-energy-nexus”. It is evident that energy and water resources will soon be very limited by adhering to conventional resource management plans.
In this paper, the authors investigated water, associated energy use, and their implications within SJSU. The “Objectives” below address specific objectives of the paper. “Literature Surveys” includes literatures on the water-energy nexus specifically focused on water distribution systems. Characteristics of water distribution systems within SJSU are explained in the section: “Characteristics of SJSU’s Water System”. “Results” discusses the results and analysis of data. Finally, the conclusion is drawn in “Analysis of Data and Discussions”.
Specific objectives of this paper are to:
Examine the available data (2006–2011) for SJSU’s main well water use and pump’s energy use to assess their correlation.
Analyze inherent reasoning for time dependent trends and variations for water and energy consumption.
- Perform economic analysis due to water/energy use change.
Water Sources and Users
The amount of water distributed and the energy required to distribute it have a dependent relationship in any water supply systems. Energy is required to move water from sources to end users, but the quantity of energy required to do so is variable. Boulos and Bros (2000) presented tactics for optimizing water distribution systems’ energy use in conjunction with assessment of carbon footprint.
They noted that decreasing the volume of water pumped (e.g., adjusting pressure zone boundaries), lowering the head against which water is pumped (e.g., optimizing supply pressures), recognizing energy tariff incentives (e.g., avoiding peak-hour pumping and making effective use of storage tanks by filling them during off-peak periods and draining them during peak periods), increasing the wire-to-water efficiency of pumps through periodic efficiency testing (e.g., ensuring that pumps are operating near their best efficiency point and replacing inefficient pumps and/or motors), andensuring proper application of variable-speed drives are good strategies to for system energy optimization.
Pelli and Hitz (2000) examined water distribution systems to assess energy savings using indicator concepts. These energy indicators enabled examine the efficiency with which the energy consumption of a water distribution system operates. They used two indicators: 1) structural indicators, which involve and are dependent on, the spatial distribution between water sources and water users, and 2) quality indicators that represent the ratio of effective energy used to transport the water over the minimal amount of energy required. By nature, energy consumption within water distribution systems is unavoidable considering losses associated with pipe major and minor losses, energy dissipation at the outlet, pressure, and elevation head changes between sources and end users. In this vein, optimization of both indicators can realize both resource conservation and financial savings. It is recommended that these indicators should be carefully considered at the initial phase of the planning and operation and maintenance phases.
According to Biehl and Inman (2010), resource conservation, system optimization, and cost savings go hand in hand, and small changes can have a big payoff. They emphasize that optimizing a water distribution system has many benefits. These include, but are not limited to, reduced operating cost, reduced greenhouse gas emissions, reduced facility energy use and cost, prolonged life of operating equipment, and overall increased sustainability of resources. Monitoring and maintaining the most efficient pumps are also a viable technique for system optimization.
Characteristics of SJSU’s Water System
The city of San Jose is located in northern California, which is the third largest city in the state and the 10th largest city in US. San Jose is considered to be the capital of Silicon Valley. SJSU’s main campus is over 154 acres and supports roughly 30,000 undergraduate and graduate students, with about 3,500 living on campus. The main campus includes about 43 buildings such as classrooms, dormitories, science labs, athletic facilities, kitchens, and cafeterias (Figure 1). The campus is capable of pulling water from the San Jose Water Company (SJWC) and South Bay Water Recycling (SBWR) to meet its demand, but meets most using its own on-campus well. Table 1 summarizes SJSU’s water distribution source breakdown.
South Bay Water Recycling (SBWR) connects with the SJSU water distribution system at the edge of campus (Martin Luther King Library, Figure 1). It is currently used in toilets and urinals at the MLK and will be used for the same purposes to the Student Union, which is currently under construction (expected to open during 2014). In addition, it supplies water to the cooling towers at the central plant. Beginning December 2011, SBWR has been used for irrigation of the campus that is around 26 acres of landscaping. There are three turnouts for the SJWC on campus, and they are located at 4th Street and San Salvador, 7th Street and San Fernando, and 9th street near to the Central Plant. All fire hydrants on campus are supplied by the SJWC. Besides the hydrants, water is supplied by SJWC only as a backup should the main well require repair, or the campus demand exceed the well’s capacity.
|Figure 2. Water versus time from November 2006 to 2011
Figure 3. Energy versus time from November of 2006 to 2011
Figure 4. Water usage versus energy usage from November 2006 to present
The main well, the campus’ primary water source, has an elevation of 91 feet above mean sea level, and the entire campus lies within the same pressure zone. The main well is located in the southeast quadrant of the campus, Village Housing, Joe West hall, and most closely, the Aquatics center (Figure 1). This well services the 43 buildings located on the main campus and the central plant through domestic service connections. The campus has pulled its own water as far back as 1940 and the main well has been serviced by the same Variable Frequency Drive pump for the past 24 years. Pressure had been maintained at about 80 psig until November 2007 when the main well shut down.
In light of the “sustainability movement” and increasing awareness of its importance, SJSU has worked to implement practices toward sustainability and conservation efforts. SJSU’s main well and pump involve two candidates for optimization—water and energy. SJSU has been putting efforts to examine the efficiency of the water distribution system. In 2008, water demand analysis and master plan was prepared. During March 2011, the MLK (public library) went online with SBWR. In December 2011 the irrigation system for SJSU also switched over to recycled water use.
The main well is equipped with meters for the purpose of evaluation and billing. The meters track the water quantity (in gallons), as well as the energy usage (in kilowatt-hours). The main well and its pump are isolated from any other water and energy consumers within the enclosure; therefore, all readings are pure reflection of the demand placed on the well and pump, and not on other extraneous sources. This relationship will enable authors to analyze relationship between water and energy usage. The meters accumulate data, and are recorded every month. Water consumption data is available dating back to January 2001, while data for the energy consumption only dates back to November of 2006. In section 5, trend for water and energy use are plotted for analyzing possible correlations between the water and energy consumption.
In an attempt to find a correlation between the water use of the main well and energy use at the pump, time series of two variables (November 2006–November 2011) are plotted (Figures 2 and 3). It is evident that two resources are closely related by similar rises and falls through five years of data. The more demand placed on main well, the more energy it is consuming.
Main well was shut down during November 2007 to December 2008 (which shows zero values), during which time SJSU met all of its water demand using SJWC. Both water and energy consumption had positive trends from 2006 until the end of 2007, when the main well was shut down. In December 2008, the main well came back online and both the energy and water consumptions have shown negative trends since then. The trends, however, have different slope values (Figures 2 and 3). Authors also plotted the relationship between water demand versus energy use (Figure 4). It is seen that they have linear relationships.
Analysis of Data and Discussions
Water distribution systems are designed for wide spectrum of consumers and facilities including commercial, industrial, residential, and firefighting. Hanemann (2007) analyzed urban water use in Metropolitan Water District of Southern California and found out that the breakdowns are residential (66.7%), commercial (16.9%), industrial (5.6%), public uses (3.6%), fire-fighting and line cleaning (2.6%), and losses and errors (4.6%). Each category of consumers and facilities has constituent factors that make up its demand/consumption behaviors. Several of these factors overlap one another, while others are unique to specific categories and/or cases within the categories.
Marella (1992) describes many possible factors contributing to variations in water demand behavior for different categories. Population and occupancy will have a large effect on most systems demands. It is obvious that the number of people utilizing a water distribution system have a direct and positive correlation to the amount of water consumed. An area’s climate will also impact the amount of water consumed. Higher temperatures tend to have a direct positive correlation, while the volume of precipitation has an inverse relation water demand.
Public attitudes about water conservation (including passive conservation, active conservation, and pure effects of price effects) and the availability of alternative water-supply sources, such as reclaimed, recycled, and desalinated, also play important roles on the demand placed on water distribution systems. Links to socioeconomic factors such as income and type of housing, as well as water-pricing practices and rate structures within the system are evident. It is quite challenging to isolate each factor, but they certainly have combined influence on water use (Tanverakul and Lee 2012).
Typically, public universities including SJSU have more dramatic shifts in water demand throughout the year than many other facilities due to genetic fluctuation in available users. During the Fall (September to December) and Spring (January to May) Semesters, student housing is occupied and assumed at full capacity, and enrollment and staffing are at their maximums. During the winter months (mid-December to mid-January), abbreviated classes are offered, but staffing, enrollment, and student living is at a minimum. The winter months are also those requiring the least irrigation, and a higher potential for precipitation. It is noted that San Jose’s wet weather starts from October to February followed by dry spell (Figure 5). The summer (May to August) also offers abbreviated classes, and while the enrollment and staffing are generally higher than during the winter, the number of users is dramatically lower than during either the spring or fall semesters. Summer in San Jose has less precipitation and more irrigation requirement than any other time in the year (Figure 5).
|Figure 5. Monthly temperature and precipitation in San Jose
Figure 6. Monthly averages of water and energy readings from November of 2006 to present
Averages of water and energy consumption (2006–2011) are illustrated in Figure 6. It is observed that the water consumption is at absolute minimum during month 1 (January). This corresponds to winter weather, little or no irrigation requirements, and very low enrollment compared to the rest of the year. The water consumption then increases almost linearly until May. This corresponds to increasing irrigation requirements due to the changing season, as well as enrollment increasing for the spring semester, and some of the highest attendance levels by May, arguably during the final stretch of the semester and finals. Following the peak in May, the water consumption decreases in June after the semester has let out, and gradually increases until September where the absolute maximum lies. The increase from June to September is indicative of the weather heating up and irrigation requirements increasing throughout the summer. Finally, in September, the occupancy spikes as the fall semester begins and high irrigation requirements are still present. Lastly, the water consumption decreases from September through December as the weather cools, the student attendance declines slightly, and irrigation requirements dwindle for the winter.
As expected, the energy profile of the pump for a year has similar trends. The exception is that the energy demand does not experience the same drop off after month five (May) when the Spring Semester finishes. A possible explanation for this is that the water fluctuation is due largely to changing irrigation requirements. The irrigation system within the water distribution supply requires relatively little energy (all of the irrigation has relatively low elevation and not as much water is requiring the energy intensive factor, i.e., pumping to the higher levels of the taller buildings), it will not have a significant decrease at this time. The energy profile instead gradually climbs from January to September, and then decreases until December.
The difference between water and energy use trend is because most of this water usage is due to irrigation in the summer months, despite having lower campus occupancy, and pumping water for the irrigation system has a lower energy requirement than that necessary for users such as students and faculty (relatively higher buildings).
The total volume of water (in cubic meters) and total energy (in kilowatt-hours) consumed by the main well at SJSU has decreased since 2008 (Figure 2 and 3). These savings should due to retrofitting aerators across campus, sustainability movement at SJSU, etc. These reductions may provide an opportunity to examine the economical savings that may be realized through further optimization, sustainability, and conservation. Table 2 summarizes the cost savings realized from decrease in water and energy consumption between 2009 and 2011. It is estimated that about $25,541 were saved in water and energy cost.
Conservation and sustainability of natural resources and environment is one of the most important issues now. Authors believe that public education sector should implement these concepts not only into their classroom education but also everyday life. Lowering water consumption within water distribution systems not only aids in the sustainability of water, but also decreases energy consumption, which helps to lessen environmental impact from greenhouse gas emissions. It was also observed that both of these reductions can lessen the financial burden incurred.
With available datasets (2006–2011) at SJSU, the trends of both water and energy consumption were analyzed. Clearly, common factors including student enrollment, precipitation, evaporation and temperature have impact on water demand. Specific behaviors in school settings’ demand variations were observed: semester based fluctuations that impact enrollment, housing occupancy, and facility use. Consumption of both water and energy is lowest during the winter months when irrigation requirements and user occupancy is at its lowest and is higher the rest of the year. Water usage experiences peaks in May and in September, due to increases in irrigation requirements and highest occupancy.
The framework developed in this research will be a good contribution to the water-energy nexus planning in University settings and research on the interdisciplinary theme of water distribution and urban sustainability. This project demonstrates an integration of the economic benefits and environmental sustainability.
Author Bios: Angela R. Ruberto is an undergraduate research assistant.
Juneseok Lee is an assistant professor of Civil and Environmental Engineering,
Adam Bayer is the director of Facility Development and Operations for San Jose State University.