Evaluation of Natural Gas to Reduce Carbon Footprint and Energy Costs for Desalination
Project: 13-05
Type: Decision Making Tool
Year Released: 2016
Program: Principal
Total Investment: $225,000 (Cash: $120,000, In-Kind cash and service: $105,000)
Principal Investigator: Mohammad Badruzzaman, Ph.D. P.E., MWH
Background
Besides energy use, the other key factor that impacts the energy costs of a desalination plant is the unit cost of power and carbon footprint. Energy costs are typically 25 to 40% of the total costs for production of desalinated drinking water. Minimizing the unit cost of power by self-generation of electricity using state-of-the art liquefied natural gas (LNG) driven gas turbine generation units is an emerging desalination industry trend. This process will allow for the harnessing of benefits for the site-specific conditions of desalination projects. The same process can be applied to large water treatment or water reclamation plants.
Goals and Objectives
The overall goal of this project is to provide a better understanding of the application of LNG/NG for self-generation of power at desalination plants and/or for the operation of gas-driven engines for large desalination plant pumping. The specific objectives of this project are to:
- Assess the application of LNG/NG at seawater desalination facilities;
- Perform an economic analysis of the application of LNG/NG for power generation at desalination facilities;
- Identify the environmental benefits/impacts of incorporating LNG/NG at desalination facilities;
- Compare the grid electricity and LNG/NG-based power generation based on life cycle cost (LCC) analysis; and
- Develop a conceptual framework for site-specific implementation of a LNG-based power generation facility at a seawater desalination plant.
Research Approach
This project team developed an Excel-based spreadsheet (LCC Tool) to conduct the cost and greenhouse gas (GHG) emissions comparison among the following power supply alternatives on a life cycle basis:
- On-site gas-fired power generation where NG or LNG is used as the sole fuel for the on-site power generation facility that powers the desalination plant;
- Hybrid systems where NG or LNG is used as the fuel for the engines that drive high-pressure pump motors in the desalination plant. The remaining energy is provided through the grid connection;
- Grid connection as the sole source that powers the desalination plant.
In order to develop the LCC tool, preliminary design and operating criteria associated with the three power supply alternatives for a number of desalination capacity sizes (within 2.5 to 150 MGD range) were developed.
The elements of the conceptual design and operating parameters of the gas engines/turbines employed to power desalination plants of various capacities and their related capital and operations and maintenance (O&M) cost information were used to conduct the LCC and life cycle greenhouse gas (GHG) emission comparison between purchasing electricity directly from the power grid and the use of commercially supplied LNG/NG for self-generation of power and/or pumping for desalination processes. The details of the tool and related guidance manual are presented in Chapter 4.
Findings and Conclusions
This study conducted a conceptual assessment of the comparative economic feasibility and GHG emissions potential of the application of LNG/NG and the electricity provided by the grid supply based on total costs (capital and O&M costs) in the first year of operation; LCC (capital and O&M LCC); levelized cost of energy; and GHG emissions (first year and life cycle).
Economic Analysis
On the basis of the conceptual cost analysis on 2.5 to 150 MGD desalination plants, the grid electricity requires lower capital investments than the LNG/NG-based options (on-site /hybrid power generation). Conversely, the grid alternative has higher O&M costs compared to the LNG-based options. Therefore, a LCC-based analysis is needed to understand the true economic benefits of the power supply alternatives.
Levelized Cost of Energy (LCOE) Analysis
An LCOE analysis was also used as a metric to compare the cost of energy generated by the different power generation options. The LCOE represents the cost per kilowatt-hour of building and operating a power generation alternative given an assumed life cycle. The LCOE analysis performed to compare the cost of energy generated by the different power source options showed that for desalination capacities of 10 MGD and higher, the LNG based on-site power generation is the lowest-cost option among all the power supply alternatives evaluated. The hybrid systems also show a lower LCOE than the grid alternative for desalination plants of capacity higher than 5 MGD.
Environmental Benefit Analysis
GHG emissions arise from electricity usage and from the possible use of alternative fuels. The GHG emission from the grid power supply is sensitive to the energy mix used in the grid and the associated emission factor. On the other hand, the efficiency of the engines/turbine impacts the GHG emission from an on-site power generation facility. The life cycle analysis for the assessment of the GHG emissions showed that when low GHG emission factors are used, such as in some areas in California under the PG&E service area, the grid electricity option resulted in lower life-cycle GHG emissions than those of LNG/NG power source alternatives. For higher emission factors, such those typically established as the U.S. national average by the U.S. EPA, the opposite was observed and the LNG-based on-site power generation appears to be the most sustainable option.
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