Developing an Adsorption Desalination Cooling System Driven by Solar and Waste Heat Using LabVIEW and PXI

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"By using NI PXI hardware and LabVIEW, we eliminated the need for a separate controller and data acquisition system, which made the system more compact. We achieved parallel synchronized data acquisition and control with minimal latency required by the dynamic nature of the system. We built the code in-house with LabVIEW graphical programming in an accelerated time frame of two months—even before we developed and assembled the hardware system."

- Pramod Kumar, Indian Institute of Science

The Challenge:
Developing a two-stage adsorber desalination cooling (ADC) system capable of simultaneously generating desalinated water and refrigeration at 7 °C using hot water from solar or any low-grade (less than 85 °C) heat as the primary energy source.

The Solution:
Using NI PXI Express and NI LabVIEW software to create an integrated solution that meets our stringent control and data acquisition requirements with the option to reconfigure the hardware in the future.

Author(s):
Pramod Kumar - Indian Institute of Science
Sourav Mitra - Indian Institute of Science
Divya Agarwal - Indian Institute of Science

The steady rise in population along with an increased standard of living leads to an exploitation of non-renewable energy resources like fossil fuels and an excessive burden on natural resources like fresh water. This is unsustainable from both an economic and an environmental perspective.

The availability of naturally occurring fresh water is limited. Research in recent years has focused on generating potable water through desalination processes with minimal dependence on fossil fuels. Commercially available technologies such as multistage flash are energy intensive and require heat input at a high temperature (greater than 120 °C) whereas reverse osmosis requires high-grade energy in the form of electricity. Thus, the current technologies in water desalination come with significant global warming potential.

An adsorber desalination cooling (ADC) system provides an environmentally conscious solution by generating desalinated water with low-grade waste heat easily obtainable from any industry or rooftop solar geyser. Figure 1 shows a broad overview of an ADC system that primarily requires heat input to run and minimal electricity input for the pumps and control valves. The ADC system also produces steam at low pressure and temperature to generate cooling. Hence, an ADC system qualifies as a multieffect system.

System Overview

Our ADC system uses steam (or water) as a refrigerant and features components similar to a refrigeration air-conditioning system except it replaces the high-grade, energy-hungry electrical compressor with a thermal compressor. The thermal compressor compresses the low-pressure steam generated in the evaporator to the high-pressure steam that is required at the condenser. Figure 2 shows a detailed schematic of the laboratory setup we built at IISc .

Evacuating the system to low pressure flash evaporates the brackish water present in the evaporator. The heat of evaporation is taken from the water flowing through the chiller coils in the evaporator, which generates cooling at that end. The system thermally compresses the low-pressure steam to the condenser pressure in a one- or two-stage system. The thermal compressor consists of four silica gel adsorber beds in each stage with copper coils passing through each of them to carry hot and cold water. The low-pressure steam from the evaporator enters these adsorber beds and goes through four stages of operation: adsorption, preheating, desorption, and precooling, which compresses it to a higher pressure. Passing hot and cold water through the copper coils and operating the steam valves in a predefined manner causes these stages to occur. Each of the four beds (per stage) in the thermal compressor goes through the sequence of four operations with a constant phase shift of 90 °C. The high-pressure steam reaches plenum then goes to the next stage of thermal compression or directly to a condenser depending on the operating conditions. The condensed water collected is desalinated water.

Instrumentation and Control

The system uses the 3/2 water valves controlled by a digital output card on a PXI system to switch between hot and cold water based on predetermined logic. In addition, each adsorber bed includes two steam valves, one each for steam inlet and outlet, which is electro-pneumatically controlled based on digital output from PXI.

The adsorber bed includes two water flow control valves to regulate the hot and cold water flow rate requiring analog output from the NI controller. The adsorber bed also features various sensors including flow, pressure, and thermocouples that provide vital information about the beds. The laboratory system simulates the solar hot water tank by using a set of electrically operated heaters to obtain a preset temperature. The system controls the pumps and heaters with the PXI digital output using a relay bank and a set of contactors plus overload relays to isolate it. Figure 3 shows the instrumentation architecture for the thermal compressor and other high-power equipment. The NI PXIe-8115 RT controller , the brain of the system, controls and monitors the thermal compressor, which is the heart of the ADC system.

We divided the NI LabVIEW software code into two distinct parts: discrete time-based valve control using deterministic timed loops and continuous data acquisition from sensors using nondeterministic while loops. We developed the LabVIEW front panel to illustrate the actual system and various states of operation so the user could see the status of each thermal compressor bed at a glance. Figure 4 shows the screenshot of the front panel for stage 1 adsorber beds. Each bed underwent a different stage of operation and was uniquely determined by the combination of Boolean logic of the valves. Similarly, the stage 2 bed provided the user with the relevant system information.

Flexible, Compact, Fast Development

We developed and tested an ADC system controlled by NI PXI Express with flexibility in the control logic and used LabVIEW to experiment and reconfigure the logic for the system. By using NI PXI hardware and LabVIEW, we eliminated the need for a separate controller and data acquisition system, which made the system more compact. We achieved parallel synchronized data acquisition and control with minimal latency required by the dynamic nature of the system. We built the code in-house with LabVIEW graphical programming in an accelerated time frame of two months—even before we developed and assembled the hardware system.

Author Information:
Pramod Kumar
Indian Institute of Science
Indian Institute of Science, C V Raman Road
Bangalore
India
pramod@mecheng.iisc.ernet.in

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