Thermal Stratification
This page discusses thermal stratification in heat pumps.
What is Thermal Stratification?
Thermal stratification is the natural phenomenon where water will separate into layers of different temperatures.
Thermocline
The thermocline is the transition region between the hot and cold portions of a stratified thermal energy storage tank.
The smaller the thermocline region, the closer the tank will be to a perfectly stratified tank. (Spielman, 2022).
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Well stratified tank and temperature profile (Spielman et al, May 2022)
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Poorly stratified tank and temperature profile (Spielman et al, May 2022)
Benefits of Thermal Stratification
Thermal stratification in a heat pump system can offer several benefits, primarily related to improving the efficiency and performance of the heat pump. Here are some of the key advantages:
- Increased Efficiency: By minimizing the HP inlet temperature, the COP is higher.
- They concluded that better system efficiency will result when there was more stratification. (https://www.sciencedirect.com/science/article/abs/pii/S2451904920302614)
- Faster Response Times for Variable Flow Pumps: With thermal stratification, the heat pump can quickly access the warmer water near the top of the tank when hot water is demanded. This leads to faster hot water recovery times compared to a non-stratified system, where the entire tank would need to be heated. *
- Extended Equipment Life: Efficient operation reduces the wear and tear on the components, potentially extending the lifespan of the heat pump.
How Does Heat Input Affect Thermal Stratification?
Heat input has the following impacts on thermal stratification:
- Enhanced Stratification with Controlled Heat Input: Controlled and gradual heat input allows for a more stable and pronounced stratification. Systems with variable heat input or modulation capabilities can adjust the input based on demand, contributing to better stratification (Lou et al., 2021).
- refer to AS/NZS 4234:2021 low flow criteria 1 L/min/kW
- Stratification Disruption with Excessive Heat Input: Excessive or rapid heat input can disrupt thermal stratification by causing turbulent mixing within the fluid. If the heat is added too quickly or unevenly, it may lead to convective currents that blend the layers of different temperatures, thereby reducing thermal stratification (Lou et al., 2021).
- Nawaz et al., [27] showed that thermal stratification can be achieved by deploying a variable speed pump for a split configuration (https://www.sciencedirect.com/science/article/abs/pii/S2451904920302614)
How Does Flow Rate Affect Thermal Stratification?
High flowrates disrupt thermal stratification since there is increased turbulence and mixing within the fluid. This turbulent mixing tends to disrupt the distinct temperature layers formed during stratification.
Furthermore, faster-moving fluid is more effective at equalizing temperatures throughout the volume, preventing the formation and preservation of thermal layers (Lou et al., 2021)..
What kind of stratification do you see in integral vs standalone HPs?
The types of thermal stratification present in integral and stand alone heat pumps are summarized in the table below.
| Integral | Stand Alone | |
|---|---|---|
| Type of thermal Stratification | Poorly stratified (BushPE & Gluesenkamp, 2016) | Moderately/Highly stratified |
| Explanation | External microchannel designed to heat up entire tank at once | Natural convection heat transfer between the working fluid and the surroundings leads to the formation of temperature boundary layers along the lateral wall of the tank. It induces thermal stratification in the storage tank (Saha et al., 2022). |