Motivation

Conventional, electricity‐powered, Direct‐eXpansion (DX) air conditioners cool the majority of existing buildings in California. Predominantly run during the summer afternoon hours, these DX systems cause significant peak power demand, with a contribution of nearly 30% of the total in California.

What is Indirect Evaporative Cooling

Indirect evaporative cooling (IEC) operates by the same fundamental concept as direct evaporative cooling (evaporating water to cool the air), except that cooling is achieved without adding moisture to the supply air stream. An indirect evaporative heat exchanger (IEHX) is the cooling source for IEC. It typically consists of a series of thin parallel plates assembled to form a multi-layer sandwich of alternating dry and wet channels. The supply air (to room) is cooled in the dry channel (without the addition of moisture) by evaporating water into the (exhaust) air stream in the wet channel and allowing the cool air in the wet channel to absorb heat from the warmer dry channel. In addition to the advantage of not adding moisture, IEC has the potential to ultimately cool the air to its dew point temperature, while direct evaporative cooling or swamp cooling is limited by the wet bulb temperature of the incoming air. The downside of an IEC is that its performance suffers in humid areas where the dry bulb and dew point temperatures are high.

One interesting option to reduce peak power is hybrid cooling – combining an indirect evaporative heat exchanger (IEHX) with a downsized DX system. The hybrid IEC/DX cooler is a promising technology, especially in dry, hot climates like the western US. For example, laboratory testing of a WCEC Western Cooling Challenge certified hybrid IEC/DX cooling system indicates almost 80% energy-use savings and over 60% peak-demand reduction, compared to conventional DX air conditioners. Among the components incorporated into a hybrid IEC/DX system, the IEHX is the critical one, as it is the core technology – the heart of the system.

Though a hybrid IEC/DX cooling system may, if it is properly designed and operated, take both advantages of the IEC (for energy efficient cooling) and the DX (for reliable cool air delivery), existing hybrid cooling systems generally pay the price of larger size and higher cost, resulting in relatively low market penetration. This means their energy saving potential is not fully tapped. Progress on development of IECs is hampered by, among other things:

  • a lack of understanding of the thermal-hydraulic behavior of the core IEHX
  • lack of a practical tool/model for analysis of differing system designs and estimation of their energy saving potential in different climates

This project aims to tap the energy saving potentials of the hybrid IEC/DX cooling system by addressing these two topics through both modeling work and experimental investigations.

Hybrid IEC/DX Systems Modeling
It is clear that the energy savings of a hybrid IEC/DX system will be heavily dependent on its design, configuration, component sizing and modes of operation, as well as the climates in which it operates. Design engineers and utilities urgently need solid, baseline savings data that can be used to analyze designs and to create incentive programs. A practical and accurate model will provide a good solution to this problem. The modeling work focuses on the behavior of the IEHX, given that it is the core of the system and there is a lack of practical models. Once the IEHX model is developed and verified, it will work as a module for system-level modeling of possible hybrid IEC/DX designs.

Thermal Modeling of IEHXs
A practical model for the steady state behaviors of plate IEHXs has been proposed, developed and compared to experimental data.The governing differential equations that describe IEHX heat/mass transfer behavior have been modified to produce a method that is analogous to the effectiveness-NTU method for sensible heat exchangers. The simplified set of equations can then be solved quickly and with numerical stability. Figure 2 shows some initial results where we compare the predictions of this model to experimental data of the IEHX developed by Davis Energy Group.

The main advantage of the proposed model is that the heat transfer performance of an IEHX can rapidly be calculated analytically with high accuracy for various IEHX air flow arrangements (e.g., counter flow and cross flow). This is desirable for a module that is embedded in a system-level hybrid IEC/DX modeling. This body of modeling work has been submitted for journal publication.

Experimental Investigations
The primary objective of the experimental investigation is to advance our understanding of the fluid flows on the IEHX wet channel surfaces in order to maximize heat transfers and minimize pressure drop, which are crucial for designing a highly effective IEHX. The experimental data will also allow further verification of the IEHX model. Since the IEHX is essentially an air-to-air heat exchanger, the pressure loss is as important as heat transfer from the point of view of effectiveness. Our initial experimental work involves measurements of the pressure drop in channels with different surface design patterns (see figure 3) including wicking surface coatings and/or pin-fin surfaces in both dry and wet conditions.

The set-up (figure 4), instrumentation, and benchmark testing of the experimental apparatus have been completed. The test uses counter current air-water flow in a single rectangular channel with a replaceable internal surface to represent the IEHX wet channel. It operates in dry conditions (with the water supply turned off) to simulate the air flow in the dry channel of an IEHX, and in wet conditions to simulate the wet channel. Figure 5 shows an example of the friction factor for a flocked/wicking surface over a range of Reynolds numbers, and a comparison with theoretical predictions. It shows a clear division of the flow regimes from laminar, through the transition region and finally fully turbulent flow. The next stage of the experiment will be to obtain and compare results for different surfaces for both wet and dry conditions. These results will feed back into the model which will be used to predict the pressure loss of different channel and material configurations.

Path Forward
The next step for the modeling work will utilize the verified IEHX model to conduct a more thorough parametric analysis to quantify which factors most significantly influence cooling performance and energy efficiency of the IEHXs so as to provide recommendations for optimizing designs. Next steps for the experimental investigation will be to study additional IEHX surface materials from existing IEHXs to account for the impact of surface patterns on IEHX hydrau¬lic performance, and continue the friction test on the IEHX surfaces in dry and wet condition to get additional data for comparison with the model.