25-08-2017, 09:32 PM
Diesel Engine Waste-Heat Driven Ammonia-Water Absorption System for Space-Conditioning Applications
ABSTRACT
This paper presents the investigation of a single-effect ammonia-water absorption system driven by heat rejected
from a diesel engine. The waste heat is recovered using an exhaust gas heat exchanger and delivered to the desorber
by a heat transfer fluid loop. The absorber and condenser are hydronically coupled in parallel to an ambient heat
exchanger for heat rejection. The evaporator provides chilled water for space-conditioning. A thermodynamic model
is developed for a baseline cooling capacity of 2 kW and a detailed parametric study of the optimized system for
both cooling and heating mode operation is conducted over a range of operating conditions. These parametric
investigations show that degradation of system performance can be limited, and improved coefficients of
performance achieved, by adjusting the coupling fluid temperature as the ambient temperature varies. With the
varying return temperature, the system is able to provide the 2 kW design cooling capacity for the entire ambient
temperature range investigated using heat that would normally be wasted by direct rejection to the environment.
INTRODUCTION
Thermally activated space-conditioning systems have several benefits. These include reduction in electrical demand
during peak utility hours, reduction in operational costs, and low environmental impact due to the use of benign
refrigerants. These advantages have led to a renewed interest in this technology for a variety of applications. This
paper investigates one such application where an absorption heat pump is implemented to use waste heat from the
exhaust gas stream of a diesel engine generator. This system leads to better utilization of source energy and is not
location limited. It could be operable in remote locations, disaster relief situations, or anywhere a diesel generator is
in operation. In all instances, the implementation of a waste heat recovery system would reduce the electrical
demand for space conditioning.
System level modeling has been performed by various investigators for a range of capacities and configurations. It
has been well established that increased system complexity will lead to improved coefficients of performance
(COPs) (Cheung et al., 1996; Engler et al., 1997; Wu and Eames, 2000; Srikhirin et al., 2001; Garimella, 2003).
Engler et al. (1997) showed that maximizing heat recovery within an absorption system leads to improved system
COPs. A basic single-effect cycle, a single-effect cycle with a refrigerant precooler, multiple absorber heat exchange
cycles, and several Generator-Absorber Heat Exchange (GAX) cycles were investigated in this study, with COPs
ranging from 0.5 for the simplest cycle to 1.08 for the most complex.
SYSTEM DESCRIPTION
A schematic of the diesel engine waste heat driven single-effect ammonia-water absorption heat pump investigated
in the present study for cooling mode operation is shown in Figure 1. The system consists of the main absorption
loop and auxiliary coupling loops for the absorber, condenser and desorber. It is assumed the evaporator is
connected to an auxiliary loop that supplies the space-cooling load. Ammonia-water is selected as the working fluid
for this investigation because the Lithium Bromide-Water pair is limited by the freezing point of water, which does
not allow heating mode operation. Lithium Bromide-Water is also prone to crystallization at high temperatures. In
addition, the high specific volume of water vapor at evaporator conditions leads to excessive pressure drops,
effectively ruling it out for compact systems.
Ammonia-Water Loop
Referring to Figure 1, concentrated solution reaches saturation in the absorber at (1). The solution is then subcooled
before exiting the absorber at (2). The solution is then pumped to the high-side system pressure across the pump,
from (3) to (4). The rectifier is cooled by this concentrated solution stream, states (5) to (6). The solution is then
recuperatively heated in the solution heat exchanger, (7) to (8), before mixing with the rectifier reflux at (9).
Concentrated solution enters the desorber where the generated vapor flows counter-current to the falling solution.
Dilute solution exits the desorber at (11) while the generated vapor exits at (12). The dilute solution rejects heat to
the concentrated solution stream across the solution heat exchanger, (13) to (14), before flowing across an expansion
device and mixing with refrigerant vapor in the absorber inlet at (28).
MODELING APPROACH
The system described above was modeled using the Engineering Equation Solver (EES) (Klein, 2010) platform.
Mass, species, and energy conservation equations were used to analyze each component in the system. As
ammonia-water is a binary mixture, three independent properties were required to establish each state point. Heat
transfer resistances were taken into account with the specification of overall heat transfer conductance UAs for each
heat exchanger. Baseline values for the UAs for each component were initially calculated by using reasonable
assumptions for the closest approach temperature (CAT) or heat exchanger effectiveness for each component. The
resulting UA values were then used as specifications for the system model. After analyzing the baseline system,
parametric analyses were conducted to maximize the system COP. System response to changes in UA values and
other key inputs was assessed to achieve progressive improvements in COP. Each parameter was varied ±15% while
the remaining inputs were held constant. Plots of system response to variations in each parameter were used to select
the final UAs and other key parameter values. The set of parameter values selected based on these analyses were
then used to understand the effect of operating conditions and other settings such as solution and hydronic fluid flow
rates on system performance. Because UA values are representative of component sizes, the UAs were used as
indicators of capital cost, and components with low sensitivities were assigned relatively lower values while still not
adversely affecting system performance.
CONCLUSIONS
A detailed investigation of a small capacity diesel engine waste heat driven ammonia-water absorption system was
performed for both cooling and heating mode applications. The primary components were hydronically coupled,
leading to a compact package using microscale heat and mass exchangers (Determan and Garimella, 2012) with
versatile implementation and installation possibilities. The baseline system can achieve a cooling COP of 0.695 at
an ambient temperature of 35°C and a heating mode COP of 1.66 at an ambient temperature of 8.33°C. System
performance was analyzed over a wide range of ambient temperatures, with different set point adjustments designed
to improve cooling and heating loads, which led to high COPs in both modes, even at extreme ambient temperature
conditions. Degradation of system performance is mitigated by adjusting water delivery temperatures, thus
maintaining high cooling or heating loads even at extreme ambient temperatures. The tradeoff of achieving
somewhat higher delivered temperatures to the conditioned space in the cooling mode, and somewhat lower
delivered air temperatures to the conditioned space in the heating mode is deemed acceptable in the interest of
maintaining high space-conditioning duties and coefficients of performance. The system under consideration here
represents one of the first small-capacity options available for waste heat recovery with high performance over a
wide range of operating conditions, and a potential for a compact envelope and low capital costs.