Large wave of water

Energy-water nexus of modern mechanical equipment

Aug. 4, 2017
How professional plumbers, HVAC engineers or designers can use this information to find the best solutions for their clients.

By H.W. (Bill) Hoffman, P.E.

Plumbing and mechanical systems have two things in common: They use both energy and water. It just depends where you start to look at this picture. The federal government and many universities have put considerable effort to quantify the relationship between water needed to produce energy and energy needed to pump, treat and distribute water, and then collect and treat the wastewater.

They coined the term energy-water nexus. Yet there is another energy — water nexus of the end user — that does not receive the same level of attention. 

The focus of this article is to examine the energy-water nexus of the end user, which, in this context, refers to equipment, appliances and mechanical systems on site that use energy and water by residential and commercial customers. The contractor — mechanical, plumbing or otherwise — works in this end-user environment.

We’ll examine the energy-water relations of four common areas that will emphasize the nexus relationships:

• Water-heating systems;

• Medical and laboratory vacuum systems;

• Cooling towers; and

• Commercial ice-making equipment.

In many cases, saving energy saves water and saving water saves energy, but then there are areas where energy savings result in more water use and vice versa.

Intertwined with this discussion is the impact rapidly rising water and wastewater costs may have in the future relative to rising energy costs. In other words, how can the contractor use this information to help clients make decisions that take changing costs into consideration?

Water, energy costs  

To begin with, let’s look at current water and sewer rates. The information in Figure 1 by the consulting firm Black and Veatch published the 2016 average water and wastewater rates for the 50 largest cities in the United States. Since then, prices have continued to climb, but this shows that water costs from 1.1 cents to 1.4 cents per gal., on average.

The last column shows these costs in cents per gallon. It is often easier to relate costs and the advantages to a customer or client in cents per gallon since they can visualize what a gallon is. For utilities that charge in hundreds of cubic feet (CCF or HCF), 100 cubic ft. equals 748 gal. 

The cost for water and sewer is only one of the factors needed to arrive at the true cost of water. Other factors include, of course, energy-related costs, the cost of any water treatment of chemicals added to the water, pre-treatment costs where applicable for such things as grease traps, additional labor associated with water treatment and wastewater pretreatment, and similar costs. 

And then there are future costs. Over the last 20 years, water and wastewater costs have experienced significant increases of an average of 5.85 percent a year, according to the Black and Veatch study. There is no indication that these increases will not continue.

By contrast, electricity costs have not risen nearly so fast, and natural gas prices have actually decreased in the last 10 years. The graph in Figure 1 comes from Michigan State University’s Institute of Public Utilities 2016 report on inflation indexes for many types of utilities, which is written by Dr. Janice Beecher and staff. 

Based on the average inflation rate found by the Black and Veatch study, water and sewer rates in 2017 have already gone up. This is backed by numerous other studies.

As the graph shows, since 1983, water and wastewater rates have risen fourfold, electricity rates by 2.2 and natural gas by only a factor of 1.6. The U.S. Department of Energy projects that electric and natural gas rates will increase at only 1.3 percent to 2 percent a year. If these inflation factors are correct, it will have a profound impact on what type of equipment and appliances are chosen.

Based national average commercial utility rates for water and sewer combined, electricity and natural gas now (2017), and just 10 and 20 years from now (2027 and 2037) are shown in Figure 2. The 2017 costs are assumed to be 5.85 percent higher than the 2016 costs stated in the Black and Veatch study.

The importance of this is that prices for water and energy are rising at very different rates. This will have profound impact on the cost of operations for the four types of systems, equipment and appliances examined and all other equipment that use both energy and water.

To put the impact of this into perspective, Table 2 shows the impact that rising water and sewer costs will have to flush a toilet in 2017 and in 2037. If inflation rates over the last 20 years for water and wastewater hold, flushing a toilet will be expensive.

Energy-water nexus and the end user

End uses of water and energy and its nexus depends on the type of use one is examining. In many cases, saving energy will also save water and vice versa.  However, there are times when saving energy will increase water use. 

Example No. 1: Hot water systems. As for the cost for heating water, the type of heater or boiler (such as condensing or high efficiency), the desired end temperature and temperature of the cold water source are all considerations for calculating energy use. How much the temperature must be raised to achieve the desired hot water temperature depends on the temperature of the potable water supply. The temperature of common water sources for water utilities generally becomes warmer the further south one is.

For example, water from the Great Lakes in the winter can be at near-freezing temperatures, while groundwater temperatures in the most southern parts of the United States can be around 75° F year around. If the desired temperature for the hot water is 140° in south Texas, one must only increase the water temperature 65°. By contrast, if the water temperature in the winter along the Great Lakes is 35°, the water temperature must be raised by 105°.

The national temperature average for tap water is about 60°. Commercial dishwashers that sanitize using heat require 180° water.

It takes one British thermal unit to increase water by 1° F.  Since water weighs 8.34 lb. per gal., it takes 8.34 Btu to increase the temperature by 1°. Table 3 summarizes the added cost per thousand gallons because of energy input needed to increase 1 gal. of water to 140° for the temperature increases indicated.

Table 3 shows, for example, that for a water source at 60° to achieve 140°, the temperature must increase by 80°. Average national water rates in 2017 are about $11.90 per thousand gal. Therefore, if the tap water temperature is 60°, the total cost for hot water would be the $11.90 for water and sewer in 2017, plus $9.52 for a gas water heater and $15.96 for an electric water heater.

This brings total cost to $21.42 for gas and $27.86 for electric water heaters — 2.14 cpg and 2.79 cpg, respectively. To put this into perspective, Figure 3 shows the percent of water within a household that is hot water by type of activity.

By 2037, combined water and sewer costs are projected to be $37.20, if historical inflation rates continue. Corresponding energy cost per thousand gallons would be $14.86 for natural gas and $32.37 for electricity, based on the Department of Energy projections. Again, based on the assumed increases in utility costs over time, 1,000 gallons of hot water (combined costs) would be $52.06 for water heated with natural gas and $69.52 for water heated with electricity

Example No. 2: Medical, dental, laboratory vacuum system. Vacuum or suction systems are found in all hospitals and many laboratories. Two generic types of vacuum pumps are the liquid ring pumps and dry mechanical pumps. A typical dental office with a liquid ring 1 horsepower pump that operates nine hours a day will use 270 gal. of water a day or more than 65,000 gal. a year as well as 1,620 kilowatt hours a year.

By contrast, dry systems use no water and are about 20 percent to 30 percent more energy-efficient. They use less than 1,300 kWh per year. Large systems such as hospitals will have two to 10 vacuum pumps, ranging from 3.0 horsepower to 10 horsepower, operating on a continuous basis. In these situations, savings will be significant if a dry system is used.

Example No. 3: Cooling towers. Chilled water/cooling tower air-conditioning systems are a mainstay for larger commercial and institutional facilities. Cooling towers take advantage of evaporative cooling. This means that the working fluid (refrigerant) can be cooled to a lower temperature in a cooling tower when compared to an air-cooled system.

Typically, cooling tower systems use 0.3 to 0.4 kWh of electricity, less than corresponding air-cooled, direct expansion systems. This means that energy savings are in the range of 3 cents per ton-hour. 

However, this does not take into account the cost of water and wastewater, cooling tower treatment cost, additional staff time required with the operation of cooling towers and other associated costs. The amount of water used per ton per hour for a cooling tower system depends on the cycles of concentration —the ratio of the dissolved solids (salt and mineral) concentration in the blowdown water divided by the dissolved solids concentration in the makeup water.

A typical operating point for a cooling tower used for air-conditioning systems is 4.0 cycles of concentration. This means that for every gallon of water the tower uses as makeup water, 0.75 gallons are evaporated and 0.25 gallons are discharged. 

Based on calculations by the American Society of Heating, Refrigeration & Air Conditioning Engineers, for every 1 ton of chiller air-conditioning capacity, 1.25 tons of cooling tower capacity must be provided since in addition to the heat removed from the building, the heat generated by the compressor, air-handling systems and water pumps is rejected to the cooling tower. 

One ton-hour is defined as 12,000 Btu. Considering the latent heat of evaporation for water, 1.48 gal. of water are evaporated for every ton-hour of cooling. For the tower, with the multiplier factor of 1.25, this is equivalent to 1.85 gal. of water evaporated in the tower for every chiller ton-hour.

Figure 4 below shows makeup requirements to the cooling tower at various cycles of concentration. If the actual tower ton-hours are known, use the blue line, but for most systems, the chiller ton-hours are to be used as the basis for estimating water use. This means that for every chiller ton-hour, a cooling tower operating at four cycles of concentration will require 2.46 gal. of water.

At the national average 2017 water and wastewater costs of $11.90 per thousand gal., the makeup water costs (water and sewer combined) would be 2.9 cents per ton-hour. Water treatment and labor costs associated with cooling tower operations adds an additional 0.1 to 0.4 cents per ton-hour. Assuming an additional treatment and labor cost of 0.2 cents per ton-hour, 3.3 cents of water-associated costs are incurred for every 3 to 3.5 cents of savings based on current utility rates. 

Some utilities offer “evaporation credits” for the water that is not returned to the sanitary sewer. At four cycles of concentration, this equals a volumetric reduction in the wastewater bill of 1.86 gal. per ton per hour, which at 2017 sewer rates would reduce the wastewater charges by 1.2 cents per ton per hour for total water and wastewater costs of 2.1 cents per ton per hour.

However, if the assumed inflation rates discussed previously occur, the electric energy savings would grow to 6 cents to 7 cents per ton-hour, but water costs per ton-hour would rise to 6 cents per ton-hour if the system receives an evaporation credit and 9.5 cents per ton-hour for total combined water, sewer, water treatment and labor costs.

This means that using a cooling tower may actually cost the facility more than an air-cooled direct expansion system. Geothermal systems eliminate water use while approaching cooling tower energy efficiencies and air variable.

Another example of the energy-water nexus of the end user notes that energy efficiency by the facility will reduce cooling tower water use by reducing the amount of heat rejected to the cooling tower. In a recent study conducted by the author, energy retrofits of lighting, using Energy Star-rated equipment and appliances and other energy upgrades, can reduce cooling tower loads by 10 percent to 20 percent.  

One new hospital in Texas uses a combined heat and power system to generate electricity and provide heat loads for hospital equipment such as sterilizers, hot water and other steam and heat requirements. It then uses waste heat to operate a desiccant system to remove humidity from the makeup air for air conditioning. This reduction in latent heat reduces heat load to the cooling towers by about 25 percent, with the equivalent reduction in cooling tower water use.

Another example of technological changes includes variable refrigerant volume (flow) air-cooled systems. Manufacturers claim that their energy efficiencies are much lower than DX systems and actually approach the energy efficiencies of cooling tower systems. The bottom line is that as water costs grow faster than energy costs, the HVAC industry will have to do its homework to develop the most efficient systems.

Example No. 4: Commercial ice machines. Commercial ice-making machines are found at restaurants, hospitals, convenience stores and similar places. These machines are rated on how many hundreds of pounds of ice they can make in a day. There are water- and air-cooled ice makers. Water-cooled ice machines use from 0.5 to 4.0 kWh less energy than comparable air-cooled models. This means that the energy savings are in the range of 4 cents to 20 cents per 100 lb. of ice made. 

However, water use for once-through cooling ranges from 75 gal. to as much as 200 gal. per 100 lb. of ice. At current water rates, water and sewer costs for water-cooled machines range from 89 cents to $2.38. In other words, in order to save 4 cents to 20 cents on energy, somewhere between 89 cents and $2.38 must be spent on water — not a bargain.

Air-cooled machines can either have outside condensers, so that the waste heat from the refrigeration process is rejected outside, or they can reject the waste heat into the heated and air-conditioned space. When the waste heat is rejected into the conditioned space, two outcomes result.

In the winter, the added heat helps reduce heating bills. In the warmer parts of the year, this heat must be removed by the air-conditioning system but even then, additional electric costs are in the 10 cents to 20 cents range. Air cooling is a winner under all circumstances.

The energy-water nexus of the end user takes many twists and turns as the professional plumber, HVAC engineer or designer looks at the best options for each situation. The fact that water and wastewater costs are increasing much faster than energy costs will have a major impact on future decisions concerning all types of equipment and appliances that use both water and energy.

We need to be ready for this challenge. Sometimes saving water will also save energy, but others times one must be traded for the other. In the latter situation, the design professional must be up to speed concerning this changing environment to make the best decisions. This is an era of changing “rules of thumb.”

Bill Hoffman, P.E., is the president of H.W. (Bill) Hoffman & Associates, LLC, a consulting firm specializing in sustainable water resource planning and water use efficiency. With more than 45 years of experience in the water conservation area, services range from policy development, to auditing and engineering studies of water conservation potential for the Construction Industry Institute sectors.

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