Thermoelectric Heat Pumps for Enhanced Thermal Energy Storage

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Thermal energy storage (TES) systems play a crucial role in integrating renewable energy sources like solar and wind power into the grid. These systems store excess thermal energy during periods of high production and release it when needed, mitigating the intermittent nature of renewables. However, achieving high-temperature storage remains a challenge for many TES technologies.

Researchers from Spain have proposed a novel TES system that utilizes a thermoelectric heat pump (TEHP) as a promising solution for achieving elevated temperatures in thermal energy storage systems ¹. Their design offers an alternative to conventional methods using variable conductance heat pipes (VCHPs) and has the potential to significantly improve energy conversion efficiency.

Understanding Thermoelectric Heat Pumps (TEHPs)

TEHPs are solid-state devices that utilize the Peltier effect to transfer heat from a cold junction to a hot junction when an electric current is applied ³. This phenomenon arises from the inherent properties of semiconductors that make up the thermal energy modules (TEMs) within a TEHP. When a current flows through these junctions, heat is absorbed at the cold side and released at the hot side. TEHPs offer several advantages over conventional heat pumps, including:

Silent operation: TEHPs have no moving parts, leading to quiet operation compared to compressor-driven heat pumps.

Scalability: TEHPs can be designed in various sizes and configurations to suit specific needs.

High reliability: The solid-state nature of TEHPs contributes to their long lifespans with minimal maintenance requirements ⁴.

However, TEHPs are also known for their lower efficiencies compared to traditional vapor-compression heat pumps. This is primarily due to the inherent limitations of current thermoelectric materials ⁵.

The Novel TES System with TEHP Integration

The Spanish research group’s design incorporates a TEHP system into a TES cycle that relies on air as the heat transfer fluid ¹. The core components include:

A thermoelectric heat pump system with six TEHP blocks arranged in a combination of one-stage (OTEHP) and two-stage (TTEHP) configurations.

An electric resistance heater for initial temperature boost.

An open TES cycle with air as the working fluid.

A fan to regulate airflow through the system.

The TEHP blocks consist of TEMs sandwiched between ceramic plates. When a current is supplied, heat is pumped from the air stream entering the system (cold side) to the air exiting the system (hot side) via the Peltier effect. The researchers employed a unique pyramidal configuration for the TTEHP stages, incorporating a highly efficient heat exchanger with water-filled heat pipes to facilitate heat transfer between the OTEHP and TTEHP stages ¹.

Experimental Evaluation and Performance Analysis

The research team built a prototype of the proposed TES system and conducted experiments under various operating conditions. They investigated the system’s performance by varying factors like:

Voltage supplied to the resistor (4 V, 6 V, 8 V, or 10 V)

TES cycle inlet air temperature (120 °C, 160 °C, or 200 °C)

Airflow rate (13 m³/hour, 18 m³/hour, or 23 m³/hour)

The findings revealed that the system achieved a maximum heating capacity of 655.5 W with a coefficient of performance (COP) of 1.35 at the highest airflow rate (23 m³/hour) ¹. This translates to an increase in air temperature from ambient conditions to 113.1 °C.

The most significant outcome lies in the system’s ability to enhance the overall energy conversion efficiency of the TES cycle. The researchers estimate an efficiency improvement of 15% to 30% for thermal energy storage temperatures ranging from 120 °C to 200 °C, when compared to traditional TES systems using electrical resistances for heating ¹. Furthermore, their analysis suggests a potential overall efficiency of 112.6% at a storage temperature of 135 °C.

Future Considerations and Potential Applications

The proposed TES system with TEHP integration presents a promising approach for achieving high-temperature thermal energy storage. The research team acknowledges the need for further investigations to fully understand the system’s behavior under varying cold source temperatures ¹. Additionally, exploring alternative working fluids and incorporating phase change materials (PCMs) for latent heat storage are promising avenues for future research.

Unlocking the Potential of TEHPs in TES

Several key areas hold immense potential for advancing TEHP integration in TES systems:

Advanced Thermoelectric Materials: Development of novel TE materials with higher efficiencies is crucial for maximizing the overall system performance. Researchers are actively exploring materials with improved electrical conductivity and reduced thermal conductivity.

Optimization Strategies: Optimizing the design and configuration of TEHP systems, including the number of TE stages, airflow rates, and heat exchanger configurations, can significantly improve efficiency and heating capacity. Computational modelling and simulations can play a vital role in this optimization process ¹.

Integration with Renewable Energy Sources: A key advantage of TEHP-based TES systems lies in their seamless integration with renewable energy sources like solar and wind power. Excess electricity generated during peak production periods can be used to power the TEHP, storing thermal energy for later use. This integration can significantly enhance the overall efficiency and sustainability of renewable energy utilization.

TES Applications beyond Power Generation: TES systems with TEHPs have the potential to find applications beyond traditional power generation. They can be employed in district heating and cooling systems, industrial process heat storage, and even for thermal management in buildings to reduce energy consumption for space conditioning.

Conclusion

The Spanish research team’s novel TES system with TEHP integration presents a significant advancement in achieving high-temperature thermal energy storage. This technology offers several advantages over conventional methods, including the potential for higher efficiencies, silent operation, and scalability. While further research is needed to optimize material properties and system configurations, TEHP-based TES systems hold immense promise for integrating renewable energy sources into the grid and facilitating a more sustainable energy future.

The article we reviewed provides a strong foundation for understanding the potential of TEHPs in TES applications. However, to gain a more comprehensive understanding of this emerging technology, it is essential to explore additional resources. Here are some key areas for further exploration:

Techno-economic analysis: A techno-economic analysis (TEA) can provide valuable insights into the economic viability of TEHP-based TES systems. This analysis would consider factors like capital costs, operating expenses, system lifetime, and potential return on investment ⁶.

Life cycle assessment (LCA): Conducting a life cycle assessment (LCA) would evaluate the environmental impacts of TEHP-based TES systems throughout their entire life cycle, from material extraction and manufacturing to operation and disposal. This assessment would help identify potential environmental concerns and opportunities for improvement ⁷.

Comparative studies with other TES technologies: A comprehensive comparison of TEHP-based TES with other established TES technologies, such as pumped thermal energy storage (PTES) or molten salt TES, would provide valuable insights into the relative strengths and weaknesses of each approach ^{8 9}.

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4. Rengel, C. J., and S. A. Klein. “Thermoelectric heat pumps for renewable energy applications.” International Journal of Refrigeration 32.1 (2009): 99-107.

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6. Dincer, I., and M. A. Rosen. Thermal energy storage: systems and applications (2011).

7. Hepbasli, A., et al. “A review of life cycle assessment studies on renewable energy technologies.” Renewable and Sustainable Energy Reviews 16.4 (2012): 1902-1911.

8. Luo, X., et al. “Numerical investigation of pumped thermal energy storage (PTES) systems used for district heating.” Applied Thermal Engineering 57.1 (2013): 140-148.

9. Li, B., et al. “A review of molten salt heat transfer for high-temperature thermal energy storage.” Renewable and Sustainable Energy Reviews 111 (2019): 891-905.

Mark Pospicil

If he is not at his desk burning through his mouse pad you can find Mark sweating on the trail (summer) or prying himself from the ice (winter) with at least one kiddo close by. Mark grew up in the southern region of the U.S. where the heat index reaches triple digits for a good portion of the year. As soon as he learned how to drive with some competence, he hit the road to embark across the U.S. and Canadian territories exploring new frontiers. Shoveling his way through mountains of snowbanks he settled down in Big Sky, MT in the Spring of 2006. Since 2006 he has worked his way down the hill to the quaint but expanding town of Bozeman, MT. Here he has worked persistently at developing relationships in various fields of construction and planning – always with a strong influence of environmental stewardship, uncompromised integrity, and graceful humility.