Combined heat and power and frozen vegetables
Large-scale energy users around the world have become aware of the benefits of combined heat and power (CHP) systems. These use a high-quality energy source—often natural gas—to generate electric power using an engine, gas turbine, or fuel cell. The heat byproduct from this electric generation is then used beneficially for process purposes, for space heating or domestic hot water, or to power an absorption chiller for process or comfort cooling.
CHP can increase overall system efficiency from 30-40% for electric generation alone to 75-80% or even higher for total energy utilization. This approach can dramatically reduce total site energy costs and cut down drastically on total carbon and other emissions.
Getting more for the energy dollar
For many industrial and institutional energy users, absorption cooling may be the key method to derive year-round efficiency through CHP. The many different types of absorption chillers all work on the same principle. In a partial vacuum environment, an absorption fluid is evaporated, removing heat from the fluid, which is then used to chill water. An external thermal source such as steam, exhaust gas or hot water is introduced to regenerate the absorption solution, allowing the fluid to continue the chilling cycle.
Where absorption works
The most frequently used system with absorption chillers uses a bromine-water refrigerant. This chiller method is ideal for comfort cooling where there is a low-cost thermal source, such as byproduct engine or turbine exhaust heat. The limitation is that it is impractical to use this cycle to generate chilled water much lower than 40°F (4.4°C) because of the risk of freezing up the machine. This temperature level is adequate for perishable product pre-refrigeration. Fortunately, another absorption system is available that offers true refrigeration into the low 30s, and far beyond into sub-freezing temperatures. That system is absorption refrigeration and freezing using ammonia-cycle chillers.
Ammonia cycle for lower temperatures
Ammonia-based absorption is not new. Many home and commercial refrigerators in the early twentieth century used this system. More recently, refrigerators for campers and remote scientific sites used this technology, employing a propane or kerosene flame as a thermal source.
The home ammonia refrigerator lost popularity because of its relatively low coefficient of performance compared to electric compression systems. However, if the thermal source is byproduct heat that would otherwise be wasted, this is not a real consideration. Here is another opportunity in the world of CHP.
The ammonia absorption system can easily produce temperatures in the range just above the freezing point of water, and can even generate refrigeration suitable for freezing product down to 0°F (-18°C) or even lower. With an adequate source of high temperature byproduct heat, it is an ideal method for using this energy productively.
Ammonia chiller packages available
Energy Concepts Company is one of the leading providers of ammonia cycle absorption chillers and related heat exchange equipment. Ellen Makar from Energy Concepts was a recent presenter at a Technology & Market Assessment Forum sponsored by the Energy Solutions Center, providing information about these applications.
Makar explained, "The colder refrigeration you want, the hotter the driving heat is required. For 10°F refrigeration, we need a heat source around 260° F that we can take down to 230°F. For -10°F we need around 280°F to take down to 250°F." She notes that engines or gas turbines can provide ample thermal energy. "Gas turbines are great sources of waste heat because of the hot exhaust." She notes that the Capstone microturbine, with an exhaust temperature of 535°F, can be a very valuable source of heat for refrigeration using an ammonia-cycle chiller.
Efficiency penalty at lower temperatures
Makar points out that with compression refrigeration, the cycle efficiency goes down as the target temperatures are lowered. "Compressor efficiency for an output at air conditioning temperatures (45°F) is about 0.6 kW per ton. For temperatures of 10°F, you need 2.0 kW per ton. With waste heat-powered refrigeration, you do not need to burn extra fuel or buy additional electric energy." She adds that the heat-source water, after it leaves the absorption chiller, can further be reused for space heating or domestic hot water.
Variety of industries use ammonia systems
Makar indicates that a variety of industries with refrigeration or freezing requirements have chosen this solution for their processes, using byproduct heat from electric generation as the source. Examples include potato processing, fruit and vegetable freezing, dairy products and meat processing. Other candidates might include cold storage warehouses, breweries, wineries, poultry processing plants and cheese plants.
Energy Concepts Company has units successfully operating at a wide range of sites, including a vegetable processor and a cheese plant in California, a fish storage warehouse in Alaska, and a public cold storage warehouse in Manilla, Philippines.
Existing generation becomes an asset
Are you a candidate for ammonia cycle refrigeration using CHP byproduct heat? If you are an industry that uses large-scale refrigeration and already have on-site electric generation and are not using the byproduct heat, it’s definitely a promising option. As Makar points out, it is practical to use refrigeration for the first stage in using heat. The chiller outlet water still contains much thermal energy that can be used for other applications.
If you are contemplating CHP, remember to include potential absorption cooling or refrigeration applications as very effective ways to use that heat and avoid the cost of compression refrigeration. Absorption is a tool to make CHP pay.
This article originally appeared in the Gas Technology Spring 2017 issue.