An Efficient Capture
January 1, 2011
Profitability and sustainability are the buzz words that define our times. However, attaining both objectives often seems like an impossible balance between two opposing poles. Is it really possible for an industrial operation to be profitable and environmentally friendly at the same time?
Before answering that question, we need to make sure we understand the meaning of sustainability. Webster’s New World College Dictionary defines sustainable as, “designating, of, or characterized by a practice that sustains a given condition, as economic growth or a human population, without destroying or depleting natural resources, polluting the environment, etc.” The U.S. Environmental Protection Agency puts it another way: “meeting the needs of the present without compromising the ability of future generations to meet their own needs.” At its most basic level, then, sustainability primarily involves the responsible use of resources - energy, water, materials of construction, food and even waste.
From that perspective, almost all of our industrial plants can move farther in the direction of sustainability while actually increasing profitability. In some cases, plants are not even aware of some of the resources they might be wasting. For example, mechanical refrigeration systems are designed to remove energy where it is not wanted and reject it to a heat sink - typically the ambient. Essentially, these systems take low-grade energy and raise it to a temperature, at which point it can be expanded. That energy is generally thought of as worthless - waste energy - when, in reality, it still has value.
Many food processing facilities that incorporate industrial refrigeration in their operations also use hot water for cleanup, washdown and a number of other applications. Often, these plants have both a mechanical refrigeration compression room and a boiler water heating room. In some cases, these two utility rooms might be separated by a single wall. Over the past few decades, there has been relatively little interest or incentive to capture the waste energy from these systems. However, with the increasing emphasis on environmental responsibility and the evolution taking place in technology, potential solutions are now available. One of these is the ammonia heat pump.
A Matter of EfficiencyIn its simplest form, the ammonia heat pump is just another stage of mechanical compression on top of a conventional ammonia refrigeration system (figure 1). On the water side, the ammonia heat pump can be placed in series with combustion boilers to preheat the water and thus reduce the amount of fuel consumed. Alternatively, it can be placed in a recirculation loop off of a hot water storage tank.
The process used to reapply the waste energy captured by the heat pump is similar to what the cooling industry has been doing every day for years. It basically involves the process of mechanical refrigeration at elevated temperatures. But instead of the chilling done on the low side, the interest now becomes the benefit received on the high side. Capacity control of the equipment is now based on the temperature of the leaving hot water. As a result of this shift in focus, the efficiency rating changes from brake horsepower per ton of refrigeration (BHP/TR) to coefficient of performance (COP), which is the measure often used to compare various methods of heating efficiency.
When water is heated, 1 BTU per pound of water is required to increase temperature by 1°F (0.6°C). This specific heating ratio, along with a boiler’s efficiency, is used to determine how many BTUs of energy must be purchased to achieve the desired level of heating. For example, many industrial water heaters are 80 percent efficient and therefore require 1.25 BTUs of energy to heat that water 1°F. With a mechanical heat pump, we assume that the heat coming into the low side is otherwise waste energy. The energy that must be purchased is the electric power required to compress the refrigerant gas. Thus, the COP will be the energy required to heat the water (the beneficial energy) divided by the energy required to operate the compressor motor. (These will need to be in the same units. A common way to make this ratio is to convert everything to kilowatts.)
A single-stage ammonia heat pump using waste energy from a typical food plant refrigeration system might have a COP in the range of 4.5 to 6.0 when heating water to the 140°F (60°C) range required for sanitation purposes. In light of the energy that must be purchased, this is substantially more efficient than a standard 75-percent efficient boiler or even a high-efficiency (95- to 99-percent efficient) direct-fired boiler.
A Positive BalanceA basic yet conservative rule of thumb is that for every 100 TR of cooling capacity in a typical food plant ammonia refrigeration system, the high side rejects enough energy to heat 35 gal/min of water for sanitation purposes. In many facilities requiring hot water, an ammonia heat pump can efficiently reapply waste heat and provide an overall savings for the plant.
For example, consider a processing facility that needs to heat 96,000 gal/day, or about 100 gal/min, of 65°F (18°C) water to 135°F (57°C). The total amount of water heated per year is 24.9 million gallons. The plant pays $10.97/Mcf for natural gas and $ .068/kWh for electricity. Using a direct-fired energy-efficient boiler, the plant would pay $158,000 each year in heating costs. Using the more common 75-percent-efficient boiler, the plant would pay $209,000 each year. However, with an ammonia heat pump, the plant would only pay $51,140 each year for the electricity for the compressor motor - a savings of 67.6 percent over the energy-efficient boiler (assuming there were no electrical demand charges).
In addition to net energy savings, reducing the amount of heat dissipated by an ammonia refrigeration system’s evaporative condensers also reduces a facility’s water consumption. Because most of the energy dispelled by these units is through evaporating water, a plant can reduce its water use by 140 gal/hr for each 100 TR of energy redirected into the heat pump. Such a reduction benefits the environment as well as the plant’s bottom line.
Potential food and beverage applications for the ammonia heat pump include fermentation process water, bottle washing, hot water for food processing as well as hot water for cleanup and sanitation purposes. In these and other applications, ammonia heat pumps can enable industrial plants to reapply the waste energy from their ammonia refrigeration systems. The bottom line is that these systems provide an environmentally friendly way to lower utility bills for a more profitable and more sustainable operation.