III. Resultados
3.2 Contrastación de hipótesis Hipótesis General
Compression–absorption (resorption) heat pump, also known as hybrid ammonia–water heat pump, combines mechanical vapor compression and absorption heat pump thermodynamic cycles.
It is well known that mechanical vapor compression heat pumps provide limited temperature lifts and absorption heat pumps have limited temperature ranges. By combining the two systems in a hybrid compression–absorption heat pump, these individual limitations can be reduced. By replacing the condenser and the evaporator of conventional mechanical vapor compression heat pumps with a resorber (vapor absorber) and a desorber (vapor generator), respectively, compression–
absorption (resorption) cycle is especially suited for heat pumping processes requiring large tem-perature lifts. It allows recovering industrial waste heat at relatively low temtem-peratures and supply heat at much higher temperatures compared to standard mechanical vapor compression heat pumps,
Edging machine
Laser 2
Laser 1
Server cooling
Space cooling
Cold water storage tank (3 m3)
Hot water storage
(1 mtank3) Chamber washing system
Drying system Five absorption heat pumps
Cooling capacity: 16 kW each Heating capacity: 34 kW each
Air-cooled Absorption
heat pump Absorption heat pump Absorption heat pump Absorption heat pump Absorption heat pump
FIGURE 1.44 Heating and cooling system with ammonia–water absorption heat pumps in a metal sheet manufacturing plant in Germany. (From IEA, Industrial energy related systems and technologies Annex 13, IEA Heat Pump Program Annex 35, Application of Industrial Heat Pumps, Final Report, Part 2, 2015b;
redrawn and reprinted with permission from IEA HPT Annex 13/35 operating agent.)
because the saturation pressures of ammonia–water mixture are significantly lower than those of pure ammonia. Consequently, high heat sink outlet temperatures are possible at moderate pressure levels compared to mechanical vapor compression heat pumps working with pure refrigerants. For example, temperatures above 100°C at the heat sink can be reached with a high-pressure level below 20 bars for ammonia–water instead of a high-pressure level higher than 62 bars for pure ammonia heat pumps.
There are two variants of compression–resorption (absorption) heat pumps: (1) with solution loop, where a liquid pump and single-phase (saturated vapor) compressor are used to overcome the pressure differentials and (2) with two-phase (wet) compression, where the compressor simultane-ously works as a vapor compressor and as a liquid pump (Infante Ferreira and Zaytsev 2002).
1.5.7.1 Single-Phase (Dry) Compression–Absorption Heat Pumps
In the single-phase (or dry) compression–absorption (resorption) heat pump, also known as the Osenbrück cycle a strong (i.e., high ammonia concentration) ammonia–water solution boils in a special heat exchanger (desorber) by absorbing heat from a (waste) heat source (Figure 1.45a). Such a system leads to large ammonia vapor superheating temperatures.
Vapor
FIGURE 1.45 Single-phase (dry) compression-resorption ammonia–water heat pump. (a) Schematic and (b) thermodynamic process of ammonia–water solution in log P−1/T diagram. C, ammonia compressor; EXV, expansion (throttle) valve; VS, vapor separator; WSP, weak solution pump.
The first theoretical comparison of this thermodynamic cycle, involving nonisothermal absorption–desorption processes, with conventional isothermal evaporation/condensation processes was provided by Altenkirch (1950) (Groll 1997).
Because the fluid exiting the desorber at state 5 as a result of the boiling process is mostly a two-phase mixture, the saturated pure ammonia vapor must be separated from the ammonia saturated liquid within a VS, prior entering the ammonia electrically driven compressor at state a via the SA. The pressure ratio across the compressor is much lower than in conventional mechanical vapor compression systems, which may result in higher COPs. The evaporation process of the strong ammonia–water mixture entering the desorber at state 4, results in a lower solution concentration and in a high ammonia vapor concentration with a significant temperature glide (Figure 1.45b). In other words, as the solution composition decreases during desorption process, the ammonia satura-tion temperature increases resulting in a mixture temperature glide.
In the ammonia compressor, the pressure and temperature of the ammonia vapor are increased, and then it enters the resorber at state b. In the resorber, the vapor is absorbed through an exothermic process by the weak solution 6 coming from the VS via the weak solution pump (process 5–5′) and the economizer (process 5′–6). The heat generated by the resorption process (6 + 1 = 2) is trans-ferred to a hot water loop, whereas the strong solution is returned back to the desorber at state 6 via the strong solution storage tank, the economizer (2–3) and the throttling device (3–4). Then, the ammonia and solution cycles repeat.
Several studies have shown that the compression–absorption heat pump exhibits some advan-tages compared to conventional mechanical vapor compression heat pumps, as, for example, (1) the possibility of varying the mixture composition to adapt the heat pump to variations in temperature levels and capacities; (2) high attainable temperatures (i.e., at least 150°C) at lower pressures com-pared to pure ammonia refrigerant; for example, saturation pressure for pure ammonia at 100°C is 62.6 bars, whereas the saturation pressure of a liquid mixture with 90% (weight) of ammonia concentration at 100°C is 54.4 bars, and the saturation pressure with 50% (weight) of ammonia is 23.6 bars; and (3) the solution temperature glides can be matched to the gliding temperatures of heat sinks and heat sources, lowering the system irreversibility. Among the disadvantages, it can be noted that any ammonia leakage will change the solution composition and the system performances, and that ammonia is a flammable/toxic refrigerant in certain circumstances.
A two-stage dry compression-absorption (resorption) heat pump cycle has been theoretically studied by Sveine et al. (1998). Theoretical calculations showed that, with a heat source at 53°C, heat can be provided at 117°C with a COP of 3.8. Moreover, the desuperheater 1, acting as an intercooler, between the compressors, has a significant impact on the system performance because, while the ammonia vapor is cooled down, thus reducing the compressors’ discharge temperatures, the liquid solution is heated to its saturation temperature.
A 7.5 MW (heating capacity) single-stage H2O/LiBr absorption heat pump with internal solution heat exchanger (economizer) has been installed in an Austrian industrial company that produces cellulose and bioenergy from raw material wood (IEA 2005b). The biomass cogeneration power plant, fired by 77% of wood and 23% of in-house solid waste, supplies steam (30 MWth) to the company, as well as electricity for about 15,000 households and heat for the local district heating network. The H2O/LiBr absorption heat pump uses as driving heat source steam from the biomass cogeneration plant at about 165°C and recovers latent heat by condensation of hot and humid flue gases exhausted by the biomass power plant (Figure 1.46). The system operates with a seasonal performance factor (SPF) of about 1.6 during a significant number of hours per year (IEA 2005b).
1.5.7.2 Double-Phase (Wet) Compression–Resorption (Absorption) Heat Pumps
The two-phase (wet) compression–absorption heat pump cycle provides higher operating tempera-tures and higher energy efficiencies (Itard 1998; Infante Ferreira and Zaytsev 2002).
The most critical component of the double-phase (wet) compression–absorption cycle is the wet compressor. It compresses a liquid-vapor ammonia–water mixture and thus has to be, as much
as possible, oil-free, and able to provide acceptable isentropic compression efficiencies. As in the single-phase compression–absorption cycle, within both desorber and resorber, the two-phase heat transfer for ammonia–water strong and weak solutions, respectively, differs from evaporation or condensation of a pure fluid, because the phase changes are not isothermal, the vapor and liq-uid phases have different concentrations, and the concentration of both phases change during the respective processes.
An experimental prototype (50 kW) with oil-free twin-screw compressor and falling film verti-cal shell-and-tube type resorber, desorber, and economizer has been designed to upgrade a water flow from 110°C to 130°C, using heat source (wastewater effluent) at 80°C (Infante Ferreira and Zaytsev 2002). Compared to a dry compression–resorption heat pump, the experimental wet com-pression–resorption heat pump achieved power input reductions, eliminated the vapor superheating and improved the system COPs by up to 20%. According to van de Bor et al. (2014), optimal per-formances (i.e., low compressor power input and short payback periods) would be obtained if the ammonia concentration at the resorber inlet is 100%.
1.5.7.3 Industrial Applications of Compression–Absorption Heat Pumps
There are many sectors where compression–absorption (resorption) heat pumps could be imple-mented, as, for example, on large metallurgical industrial platforms aiming at recovering low-temperature heat from air compressors to heat domestic hot water for large district networks (Pop et al. 1983; Minea and Chiriac 2006). To recover heat from high-temperature combustion gases, ammonia–water hybrid absorption–compression heat pumps could be competitive if the maximum concentration of ammonia is kept between 0.7 and 0.9 (Ommen et al. 2011). Spray drying processes, as large energy consumers, traditionally using fossil fuels as primary energy sources to provide high amounts of greenhouse gas emissions are also eligible for improving their energy efficiency by using (dry or wet) compression–absorption heat pumps. This is mainly
SHX
Condensate Qgen
Qabs Qeva
Qcon
Driving source: steam from biomass plant
Flue gas from the biomass plant
Exhaust gas
FIGURE 1.46 Process flow sheet of the single-stage H2O/LiBr absorption heat pump for the flue gas con-densation of an Austrian biomass plant. (From IEA, Industrial energy related systems and technologies Annex 13, IEA Heat Pump Program Annex 35, Application of Industrial Heat Pumps, Final Report, Part 2, 2015b redrawn and reprinted with permission from IEA HPT Annex 13/35 operating agent.)
because the implementation of conventional mechanical vapor compression heat pumps in such an industrial sector is technically restricted by the excessive high temperatures of the exhaust air (80°C–100°C). The availability of newly developed high-pressure ammonia compressors and components (i.e., up to 52 bar maximum pressure) (see Section 1.3.2) in combination with the compression–absorption heat pump technology allows supplying heat at temperatures up to 150°C in industrial processes as processes such as spray drying. However, the actual economic and environmental savings should be estimated for each particular application (Brunin et al.
1997, Jensen et al. 2014a, b).
A detailed theoretical study on technical, economic, and environmental implications of imple-menting ammonia–water hybrid absorption–compression heat pumps in energy-intensive spray drying industrial processes used for dry solids, as powdered milk, detergents, and dyes from liquid feedstock has been recently reported (Jensen et al. 2014a, b). In the proposed application, 100,000 m3/h of ambient fresh drying air is heated from 20°C up to 200°C (by using a gas-burned air heater as a backup heat source) by recovering 6.1 MW of thermal power from the exhaust air leaving the spray drying chamber at 80°C. When the air reaches the target temperature of 200°C, it enters the spray drying chamber and is mixed with the atomized stream of the liquid product. This evaporates the liquid from the product and, then, the dry product is extracted from the bottom of the drying chamber. The exhaust air leaves the drying chamber at 80°C and a MC of 0.045 kg/kg.
Half of the exhaust air stream is used as heat source for the ammonia–water hybrid absorption–
compression heat pump. Inside the resorber heat recovery closed loop, the heat transfer fluid is water. It was theoretically found that an 865-kW ammonia–water hybrid compression–absorption heat pump with an ammonia mass fraction of 0.81 and a circulation ratio of 0.45 could reduce the facility’s CO2 emissions by 210 tons/year (Jensen et al. 2014a, b).
Among the most promising applications for compression–resorption heat pumps with high tem-perature lifts are distillation columns, which can generate half of the operating costs of petrochemi-cal plants. Typipetrochemi-cally, the temperature of distillation columns is low at the top and increases when moving to the lower sections, reaching a maximum in the re-boiler. In distillation, two or more components are separated based on their difference in boiling point. A mixture is boiled up in the re-boiler of a column, stripping off most of the light component and a part of the heavy component.
The remaining flow leaves the column as product stream. The vapor created is relatively rich in low boiling component and moves toward the condenser in the top of the column. Here, the over-head vapor is condensed and leaves the system as product, or flow back into the column as reflux.
Between the re-boiler and condenser there are trays to increase the separation efficiency. Again starting at the re-boiler the vapor moves up each tray, loses some of the heavier component, there-fore becoming richer in the light component. The liquid flowing downward becomes richer in the heavy component. The temperature gradually decreases from the bottom of the column to the top of the column (van de Bor et al. 2014). Such installations achieve low thermodynamic efficiency (e.g., about 12% for crude distillation), requiring high qualities of energy in the re-boiler, while reject-ing a similar amount of heat in the condenser, at lower temperature. Compression–resorption heat pumps can thus be used to upgrade the low quality energy in the condenser in order to upgrade the condenser heat to re-boiler temperature level and thus reduce the consumption of valuable utilities.
By matching the absorption cycle with the heat loads of distillation columns it is possible to mini-mize the consumption of primary energy (van de Bor et al. 2014).