Phase-Change Materials for Hydronic Systems Update
This project aims to reduce the running costs of hydronic heating and cooling systems in commercial buildings by reducing the pumping power. Pumping power can be reduced by lowering the required flow rate of the heat transfer fluid. However, maintaining capacity while reducing the flow rate requires an increase in the heat capacity of the fluid. Ice slurries have been used to do this in cooling applications, but while ice has a high latent heat of fusion (334 kJ/kg) and effectively increases the heat capacity of water, its use is limited by its fixed melting temperature. In order to address this limitation, we are investigating the use of phase change materials (PCMs) in hydronic systems.
PCMs have great potential for thermal energy storage. There are a wide range of applications of PCMs, ranging from building materials to clothing. The advantage of using PCMs rather than ice is that they can be engineered to melt at any desired temperature which facilitates their integration into existing systems.
The most promising PCMs are based on paraffin waxes due to their combination of a wide range of melting temperatures, high latent heats, low cost, and chemical stability.
To use these waxes in hydronic systems, we need to know how they behave when mixed with water. The simplest way is to create an emulsion, but this is likely to result in the wax depositing on the pipes, valves, and pump, causing loss of efficiency and capacity, as well as possible equipment failure. So the PCM needs to be physically isolated from the fluid. This is accomplished by encapsulating it.
Microencapsulation is generally classified as encapsulation of a substance with a diameter between 1-100 microns. Larger than this and the particles are considered to be macroencapsulated. The benefits of PCM microencapsulation in hydronic systems are multiple:
- 1. The smaller size helps maximize the surface to volume ratio thereby reducing the time it takes for the particles to melt or freeze. This is particularly significant in view of the poor thermal conductivity of solid paraffin wax.
- 2. Smaller particles are less likely to be trapped by pump impellors or valves, and so are less likely to rupture and release paraffin into the water, leading to potential failure
Behavior of Slurries
A mixture of microencapsulated PCM and water will act like a slurry. The increased viscosity of the slurry may offset some or all of the pump energy savings due to the lower flow rate, and the heat transfer behavior will be different from pure water. We have modeled the behavior of PCM slurries with varying fractions of PCM. Typical behavior is shown in the graph to the left where we have modeled a 4 ton system.
The reduction in pumping power is easy to see. The sharp drop in slurry flow rate and pumping power at a concentration of ≈ 0.08 is due the transition from laminar to turbulent flow. The model uses data for a commercially available microencapsulated PCM (made by Microtek) which has a melting point of 133ºF and a latent heat of 73BTU/lb.
Hydronic System Model
The goal of this experiment is to determine the performance of PCMs in a hydronic system. We are building a test set up in our laboratory which will replicate the behavior of a 2 ton heating and cooling system, shown in the PCM schematic on the left. A supply of conditioned air will provide a load to a fan coil, and a water to slurry heat exchanger will regenerate the PCM. The experiment will consist of 4 phases. The first phase will consist of pumping pure water to determine the pumping power required if the system did not include PCMs. The second phase consists of testing the slurry under heating conditions using a PCM with a phase change temperature of approximately 140°F. Then a different PCM with a lower phase change temperature (~50°F) will be used to create another slurry which will be tested under cooling conditions and finally a slurry containing PCMs with both high and low melting points will be used to simulate a 2 pipe system. The slurry will pass through a loop where it will go through a conditioning phase in the first heat exchanger. In the second heat exchanger the slurry will exchange heat with air which will be delivered to the room. The air will also need to be pre-conditioned to meet the standard temperatures for HVAC testing. Cold and hot water supplies on site will be used to pre-condition the slurry and the outdoor air.
During this experiment we will subject the PCM microcapsules to both thermal cycling and mechanical stresses. In order to separate any effects, we will also conduct two other experiments in which we subject the PCM capsules to only one variable: thermal or mechanical stress.
To determine the mechanical stability of the PCMs, a test will pump the PCM slurry in a closed loop. Samples will be taken periodically and studied to determine if any capsules have ruptured. The samples will be photographed, and imaging software will be used to determine the range of diameters before and after the pumping test. This will give an indication as to whether different sizes of capsules are more susceptible to rupture than others.
Thermal Cycling experiment
In order for the PCMs to remain useful in our application, their thermal properties need to remain constant after repeated cycling. The literature suggests that paraffin based PCMs are generally thermally stable. A thermal cycling test will measure the change in thermal properties such as melting temperature and heat of fusion, after repeated cycling. A sample of the slurry will be placed in a sealed container and thermally cycled between temperatures 10°F above and below the phase change temperature. Samples will be periodically removed from the container for testing by Differential Scanning Calorimetry.