Condensing gas-fired hydronic boilers have become increasingly popular in commercial heating applications during the last 20 years. Several models of condensing boilers now have thermal efficiencies approaching 100 percent at low firing rates and return-water temperatures. However, the 100-percent level remains the fundamental efficiency limit for hydronic heating systems in the commercial-buildings sector.

Gas-fired absorption heat pumps can be used in commercial heating applications and achieve thermal-efficiency levels as high as 145 percent by drawing heat from renewable energy resources, such as the ground, water, or ambient air. Absorption heat pumps use natural ammonia-water refrigerant and offer energy and carbon-dioxide- (CO2-) emissions savings of up to 30 percent or more, compared with condensing boilers.

Absorption heat pumps have gained significant market share in Europe and recently were introduced to North America. They offer new opportunities to significantly improve energy efficiency in schools/universities, hospitals, office buildings, and manufacturing facilities.

The Absorption Thermodynamic Cycle
The absorption thermodynamic cycle incorporates several stages of temperature and pressure change that enable ammonia-water working fluid to absorb heat from a gas-fired burner, ambient air, or geothermal heat sources and transfer it to a hydronic distribution loop.

• The first stage of the absorption cycle includes heating the working fluid in a gas-fired-generator component. The ammonia portion of the working fluid boils into vapor under high pressure and passes through a partially cooled rectifier to remove residual water.
• During the second stage of the cycle, the ammonia vapor enters the condenser, where it is changed back into a liquid state while transferring heat to the hydronic loop.
• During the third stage, the ammonia liquid passes through a throttling device and undergoes a substantial pressure reduction.
• During the fourth stage, the low-pressure liquid ammonia enters the evaporator component and changes back to vapor as it absorbs heat from the air or geothermal/water heat source.
• During the fifth stage, the ammonia vapor continues to the absorber component, which contains a weaker solution (i.e., a lower ammonia concentration) of the original ammonia-water working fluid, and is absorbed into the working fluid through an exothermic reaction that increases the temperature of the working fluid and releases additional heat into the hydronic loop.
• During the sixth stage, the liquid-solution pump increases the pressure of the ammonia-water working fluid to meet the high pressure of the generator component. The absorption cycle then repeats itself, beginning with the first stage.

Certain analogies between the absorption cycle and the compressor-driven Rankine cycle commonly used in electrically driven heat-pump systems exist. The liquid-solution pump and gas-fired-generator components perform the same function as the compressor in the Rankine cycle by producing high-pressure ammonia vapor. Also, the absorber component draws the ammonia vapor from the evaporator component and is analogous to the suction side of a refrigerant compressor. Finally, the throttling device and evaporator component in the absorption cycle perform the same pressure-reduction and heat-absorption functions as in the Rankine cycle.

For each unit of fuel used in the gas-fired-generator component, 0.85 units of heat are transferred directly through the condenser component into the hydronic distribution loop. Additionally, the ammonia refrigerant draws up to 0.60 units of low-temperature heat from the ambient air or geothermal/water source while in the evaporator component and transfers it to the hydronic distribution loop during its absorption into the weak ammonia-water solution located in the absorber component. Therefore, a total of 1.45 units of heat can be transferred into the hydronic loop for each unit of fuel input, thus achieving the described overall thermal efficiency of 145 percent.