Many of the greatest sustainable design opportunities lie within the central utility plant. This article discusses a range of design concepts, including site selection, energy and water conservation, emissions, alternative fuels, materials selection, and innovation.

Most central plant facilities have opportunities for sustainable upgrades with modest initial investment and great rates of return, as well as opportunities for larger-scale investment. Here is a broad menu of concepts, from relatively easy retrofits for existing plants to others that would require integration into the initial design process.

SUSTAINABLE SITES

Given the emissions, noise, and fuel storage associated with a typical central utility plant (CUP), it is particularly important to consider sustainable design issues in the siting of a central utility plant. Sustainable CUP design should incorporate the following strategies:

* Avoid sites on prime farmland, elevations less than 5 ft above 100 ft year flood plain, or sites within 100 ft of water or wetlands.

* Use a "brownfield" site, if practical.

* Consider the impact of emissions on adjacent buildings, use wind tunnel analysis to evaluate alternative sites and emissions technologies.

* Consider the noise impact of the CUP on adjacent buildings, locate away from residential areas.

* Route distribution piping through planned utility corridors, along streets, and existing pathways. Avoid disruption to undisturbed sites and vegetation.

* Locate the plant in proximity to the load, minimizing distribution energy and capital costs.

* Centralize fuel storage in a common location to serve generators, boilers, etc.

* Utilize sites that allow 100% of the liquid waste discharged from the building to flow by gravity (avoid pumped systems).

The plant should ideally be located with sensitivity to its neighbors. It should be located so that it is distant from and downwind of any critical air intakes. Wind tunnel analysis of the site is highly recommended to predict impacts of emissions on the local microclimate. Any adverse effects can be identified early and mitigated by alternative siting or application of design alternatives. The noise impact of the plant should also be evaluated, with particular emphasis placed on the distance from residential areas.

WATER EFFICIENCY

Most of the water used in a typical CUP is related to cooling tower operation. Here are a few notable methods for saving water.

Cooling towers represent the largest water users in a typical plant. The combination of evaporation, blowdown, and drift at a cooling tower will result in 36 gpm of water lost per 1,000 tons of cooling (assuming three cycles of concentration and a 10[degrees]F condenser water [DELTA]T). There are many ways to reduce or eliminate this water loss. One means is by the use of onsite water sources, such as collected rainwater, graywater, or subsoil drainage water (if available). Alternative water treatment methods such as ozone treatment or pulsed magnetic technology will often allow an increase in the allowable cycles of concentration, thereby reducing the blowdown component of cooling tower water use. Alternative water treatment technologies offer the added benefit of either the reduction or complete elimination of chemical use.

One of the best ways to reduce cooling tower water consumption is to use an alternative method of heat rejection. The use of a heat sink completely eliminates the water loss associated with cooling tower use, and will typically reduce the energy consumption associated with heat rejection. Potential heat sinks include process cooling water loops, and heat pump cooling water loops, as well as innovative heat sinks such as sewer system heat exchangers or ground source loops.

On the heating side, the use of a bottom blowdown tank can eliminate the cooling water associated with the use of a bottom blowdown cooler. The tank will be considerably larger than the cooler, but will pay back over time, particularly if boiler blowdown is more frequent than once per day, or where water costs are high (Table 1).

ENERGY AND ATMOSPHERE

The four major sustainability categories under the energy and atmosphere heading are:

* Optimize energy performance;

* Emissions reduction;

* Refrigerant selection; and

* Commissioning.

Let's start by looking at several options for optimizing energy performance before moving on to the other categories.

ENERGY-EFFICIENT BOILERS

Firetube boilers, particularly three-pass and four-pass designs, can be 3% to 5% more efficient than industrial watertube boilers. If the higher pressure capability of a watertube boiler isn't necessary, consider firetube boilers. Firetubes most often yield a lower lifecycle cost than their watertube counterparts. Currently, most industrial watertube boilers just meet the code minimum 80% efficiency for gas-fired equipment. Firetube boilers are available with efficiencies as high as 85%.

The DOE launched its "Super Boiler Project" in 2000. The goal of the project is to leverage existing and emerging technologies to create a boiler that is 94% efficient while generating no more than 5 ppm NOx and 5 ppm C[O.sub.2], by the year 2020. As this program advances, it is expected that commercially available boiler burner efficiencies will rise toward the target levels (and emissions levels will fall toward their 2020 target levels).