Ammonia-water Absorption System Based on Plate & Shell Heat Exchangers

Due to the increasing price of scarce fossil fuels, there is a worldwide focus on reducing energy consumption. This can be achieved through high efficiency technologies, renewable energies and decentralised power generation.

In Germany, a renewable energy law (EEG) was enacted in 2000. Since then, numerous projects in the sectors of solar, wind, biomass and even geothermal energy have been implemented. The proportion of renewable energy used should reach 20% of the total national energy supply by 2020. The law regulates the financial return received for electrical energy that is produced by cogeneration of heat and power (CHP), for example.

Since the implementation of the EEG, the number of CHP plants has increased significantly. The thermal energy is used locally, substituting fossil fuels such as oil or Natural Gas. Unfortunately, the capacity and the demand are for the most part unequal. With most applications, a part of the CHP’s heat output is emitted to the atmosphere as waste heat, especially in the summer, when demand decreases considerably.

To resolve this situation, an innovative use of heat can be employed: ammonia-water absorption refrigeration technology. Combined with a conventional CHP system, it forms a complex local energy system producing cold, heat and power (CCHP), which is known as trigeneration.

Shell & tube heat exchangers are usually preferred in absorption systems, but semi-sealed, sealed or fusion-bonded plate heat exchangers, coaxial heat exchangers, or Plate & Shell heat exchangers could also be used. Most importantly, the chosen heat exchanger should be compact and durable, economically mass-produced and free from leakage problems.

The absorption refrigeration technology is driven by heat input rather than electrical power input. For applications at 10–20°C, such as comfort cooling, the cooling energy can be supplied by means of absorption refrigeration technology like desiccant evaporative cooling systems, periodical water/silica gel or water/zeolite adsorption systems, and water/lithium-bromide absorption systems. These chillers are widely available even for small refrigeration capacities, while the need for solutions allowing for cooling between 0°C and lower with cooling capacities between 30 kW and 1000 kW are met.

For a long time, suitable technologies that could meet the demand economically were not available Therefore, an ammonia/water absorption system has been technically defined and has been evaluated both thermodynamically and economically. The system consists of Plate & Shell heat exchangers, a solution pump, valves, filters, controlling equipment and safety devices. It is designed for refrigeration loads of up to 1 MW. The driving heat input can be applied at temperatures down to 90°C. The coefficient is defined by the ratio of the refrigeration load and heat input. It is lower than that of a compressor chiller driven by electrical energy. The COP of a real sorption chiller varies from 0.3 to 0.6, depending on the temperatures of external heat transfer media.

As an example, a heat input of 2370 kW is required at 100°C to operate on a 950 kW refrigeration load at -10°C. An additional 28 kW electrical power input is required for the solution pump.

Zimmermann GmbH acts as general contractor supplying high efficiency CHP units, compressor chillers for the deep-freezing sector, hybrid recoolers, piping and surrounding equipment. Seasonal temperature fluctuations, and in particular the alternating operational demand of cold, heat and power, have led Zimmermann to develop an intelligent, individually adaptable controlling system to carry out this sophisticated task.

Dr Wolfgang Stürzebecher

Zimmermann GmbH