Absorption Heat Pumps

Absorption heating/cooling systems enable high-efficiency utilization of low-grade thermal sources such as solar-thermal, waste heat, geothermal and low-grade biomass combustion to provide heating and cooling for small-scale (residential and mobile) applications. During my graduate research at Georgia Tech (2012-18), I worked on the development of compact ammonia-water absorption heat pumps and their control systems. The following projects provide a glimpse of my PhD research.

Microscale Absorption Chillers for Residential and Mobile Applications

During my PhD, I worked on two prototype systems: a gas-fired residential chiller (3.5 kW cooling capacity), and a waste-heat driven chiller (2.6 kW capacity) for mobile applications. These systems employed micro-scale heat exchangers to enable modular and compact systems, at the same time providing high heat and mass transfer coefficients. Automated operation of these systems demonstrated stable performance over a range of operating conditions. In addition, I fabricated a breadboard test facility to perform a detailed analysis of absorption system controls. The experimental facility provided full functionality to implement a control system with variable actuators such as valves, pumps, and heat source/sink conditions.

Publications

1. Garimella, S., Keinath, C. M., Delahanty, J. C., Hoysall, D. C., Staedter, M. A., Goyal, A., Garrabrant, M. A., 2016. Development and demonstration of a compact ammonia-water absorption heat pump with microscale features for space conditioning applications, Applied Thermal Engineering, 102, pp. 557-564. DOI: http://dx.doi.org/10.1016/j.applthermaleng.2016.03.169 (PDF)

2. Goyal, A., Staedter, M. A., Hoysall, D. C., Ponkala, M. J., Garimella, S., 2017. Experimental evaluation of a small-capacity, waste-heat driven ammonia-water absorption chiller, International Journal of Refrigeration, DOI: http://doi.org/10.1016/j.ijrefrig.2017.04.006 (PDF)

3. Boman, D. B., Hoysall, D. C., Staedter, M. A., Goyal, A., Ponkala, M. J., Garimella, S., 2017. A method for comparison of absorption heat pump working pairs, International Journal of Refrigeration, DOI: http://doi.org/10.1016/j.ijrefrig.2017.02.023 (PDF)

4. Garimella, S., Ponkala, M. J., Goyal, A., Staedter, M. A., 2019. Waste-heat driven ammonia-water absorption chiller for severe ambient operation, Applied Thermal Engineering, DOI: https://doi.org/10.1016/j.applthermaleng.2019.03.098 (PDF)

Control of Compact Microscale Absorption Chillers

I developed control algorithms for ammonia-water absorption heating/cooling systems. Working with a dynamic model of an absorption system, I implemented feedback control algorithms on a prototype absorption chiller. Simple feedback algorithms for controlling the heat input rate in the system and valve positions demonstrated an increase in system efficiency at part-load and off-design operating conditions. Finally, I developed and implemented a multivariable feedback controller to optimize the part-load performance of the chiller.

Publications

1. Goyal, A., Rattner, A. S., Garimella, S., 2015. Model-based feedback control of an ammonia-water absorption chiller, Science and Technology for the Built Environment, 21 (3), pp. 357-364. DOI: http://dx.doi.org/10.1080/10789669.2014.982412 (PDF)

2. Goyal, A., Garimella, S., 2019. Multivariable feedback control of small-capacity ammonia-water absorption systems, Energy Conversion and Management, DOI: https://doi.org/10.1016/j.enconman.2019.03.080 (PDF)

3. Goyal, A., Staedter, M. A., Garimella, S., 2019. A review of control methodologies for vapor compression and absorption heat pumps, International Journal of Refrigeration, 97, pp. 1-20, DOI: https://doi.org/10.1016/j.ijrefrig.2018.08.026 (PDF)

Generalized Transient Simulation Methods for Thermal Systems utilizing Zeotropic Working Fluids

Control design for absorption system involves dynamic modeling of the system to predict the evolution of system properties and performance with time. My research focused on developing computationally efficient transient models of absorption systems using the moving-boundary method. These models provided 60x computational speedup over conventional discretized models and are more suitable for control system design.

Publications

1. Goyal, A., Garimella, S., 2018. Generalized transient simulation of two-phase heat exchangers using zeotropic fluid mixtures, International Journal of Refrigeration, DOI: https://doi.org/10.1016/j.ijrefrig.2018.07.031 (PDF)

2. Roeder, A. A., Goyal, A., Garimella, S., 2019. Transient simulation of ammonia-water mixture desorption for absorption heat pumps, International Journal of Refrigeration, DOI: https://doi.org/10.1016/j.ijrefrig.2019.01.032 (PDF)

Fast Computation of Thermodynamic Properties of Ammonia-Water Mixtures using Neural Networks

Calculation of thermodynamic properties of mixtures can be a difficult computational problem using the typical equation of state (EoS) methods. For zeotropic mixtures, the thermodynamic states are highly non-linear functions of the independent properties used to compute them, and the resulting equations require an iterative solver for the calculation of each state point. Moreover, the accuracy of EoS formulation and mixing rules becomes poor with increasing polarity in the constituent species of the mixture.

In this project, I used artificial neural networks (ANN) to compute the thermodynamic properties of ammonia-water mixture used in vapor absorption heat pumps. The error in the computations using this method is less than 0.1% in comparison to the EoS method for a wide range of operating parameters. I used the new property calculation modules to simulate the dynamic heat transfer response of a representative ammonia-water condenser in a vapor absorption cycle. It is observed that the ANN-based condenser model is computationally faster by 80% in comparison to a model usign EoS-based thermodynamic property calculations.

Publication: Goyal, A., Garimella, S., 2019. Computing thermodynamic properties of ammonia-water mixtures using artificial neural networks, International Journal of Refrigeration, DOI: https://doi.org/10.1016/j.ijrefrig.2019.02.011 (PDF)