The Water Energy Nexus and Biopower Production
A recent workshop on water and energy confirmed my understanding of the dynamic relationship water and energy share. Although the fossil power industry is becoming less reliant on large water resources because of the advent of more efficient, lower cost, air cooled and hybrid air/water–cooled condensers used in cooling boiler intake water, the need for large volumes of water still exists for the foreseeable future. For biomass power systems, the same cooling and other peripheral requirements for water still exist, but in many cases—especially for energy crops and agricultural residues—there is the added water balance requirement of agricultural water used in growing the green, renewable fuel. Though the low cost of natural gas seems to be the greatest challenge for biomass power development right now, water adds another challenge.
The power industry is second only to agriculture as the largest domestic user of water, accounting for 39 percent of all freshwater withdrawals in the nation, 71 percent of which is used in fossil fuel-based electrical generation. The same technologies used to produce electricity from fossil-based fuels are, and will continue to be, used for a significant amount of biomass-based power production. Therefore, biopower systems are going to be challenged in obtaining site permits for new biomass power plant construction. The availability of water for use in biomass electric power generation may be limited in many parts of the U.S., and biomass power plants must compete with other industrial customers, agricultural interests and households for this limited commodity. Difficulty in obtaining necessary water permits can lead to delayed or abandoned projects.
Infrastructure needs may also create a challenge with respect to water and biopower. A system for sustainable water supply can take years to develop with today’s entanglement of water rights and laws. Usually, these types of water rights issues are settled in court—over 90 percent—as opposed to the conference room. In areas that do not have an adequate water source, biomass power plant construction is often not even considered, even though these locations are ideal in other respects. Additionally, potential regulations curtailing carbon dioxide emissions will impact water use. Because of the corrosive nature of carbonic acid, water will need to be removed to very low levels prior to the carbon dioxide being pipelined to its final destination.
In lieu of these challenges, all hope is certainly not lost, as demonstrated by many biopower projects that are moving forward. Along the lines of water savings and efficiency, the Energy & Environmental Research Center, in conjunction with several commercial partners, is investigating several water-saving technologies. Tremendous new strides are being made in air-cooled condensers, hybrid air-cooled systems, water capture/recycle, and novel heat exchange media for hybrid cooling tower systems.
One example is a hybrid wet/dry cooling system that utilizes a direct-contact, jet spray condensing cycle that is air-cooled in conjunction with a conventional wet cooling loop. This system can dramatically reduce water use and also has the potential to be retrofitted into existing plants. Retrofits in existing systems can be particularly difficult for conventional dry or hybrid systems because of space limitations required for modifications at the condensing site after the turbine and the required footprint needed for air-cooled systems on the grounds, as well as all of the requisite ducting.
Another example currently under development at the EERC is a novel dry cooling technology. The system uses a nonvolatile heat-transfer fluid that takes advantage of primarily sensible heat rejection and only minimally relies on the latent heat of evaporation. The end result is a great reduction in water input.
These are just a few examples of how solutions are being found to reduce the overall water footprint of heat and power production utilizing biomass. The biomass industry can play a part in reducing the water footprint of biomass utilization systems, whether it is in power production or in the production of bioproducts or biofuels, and the EERC is working with industry to do so.
Author: Bruce C. Folkedahl
Senior Research Manager, EERC