Waste Heat to Power: Full‐Cycle Analysis of a Thermally Regenerative Flow Battery

Large amounts of waste heat, below 120 °C, are released globally by industry. To convert this low-temperature waste heat to power, thermally regenerative flow batteries (TRFBs) have recently been studied. Most analyses focus on either the discharging or the regeneration phase. However, both phases have to be considered to holistically assess the performance of the flow battery. Therefore, a dynamic, open-access, full-cycle model of a Cu–NH3 TRFB is developed in Modelica and validated with data from the literature. Based on the validated model, a trade-off between power density and efficiency is shown that depends only on the discharging strategy of the flow battery. For a sensible heat source with an inlet temperature of 120 °C and heat transfer at a thermodynamic mean temperature of about 90 °C, the power density reaches 38 W m−2 over a complete cycle, and the efficiency reaches 20% of Carnot efficiency. In a benchmarking study, the power production of the flow battery is shown to already achieve 34% of a fully optimized organic Rankine cycle. Thus, TRFBs require further optimization to become a competitive technology for power production and energy storage from low-temperature waste heat

Experimental Validation of a Dynamic Adsorption Chiller Model Using Optimal Experimental Design

A potential way to reduce the global greenhouse gas emissions for cooling and heating are thermally-driven cooling and heating technologies, such as sorption chillers. However, sorption chillers often suffer from high electricity consumption due to sub-optimal system integration. To optimize system integration, model-based approaches are very promising, but require valid models. For efficient model validation, we present a method based on the Optimal Experimental Design (OED) and apply the method to a dynamic adsorption chiller model. For this purpose, we plan an experiment with OED, execute it and estimate the unknown model parameters. To improve parameter accuracy and thus a valid model, we repeat the procedure iteratively. The results show that the presented method leads to a valid adsorption chiller model with only three optimally planned experiments. Furthermore, we show that the experimental effort decreases by up to 83 % compared to randomly planned experiments

Waste Heat to Power: Full-Cycle Analysis of a Thermally Regenerative Flow Battery

Large amounts of waste heat, below 120 °C, are released globally by industry. To convert this low-temperature waste heat to power, thermally regenerative flow batteries (TRFBs) have recently been studied. Most analyses focus on either the discharging or the regeneration phase. However, both phases have to be considered to holistically assess the performance of the flow battery. Therefore, a dynamic, open-access, full-cycle model of a Cu–NH3 TRFB is developed in Modelica and validated with data from the literature. Based on the validated model, a trade-off between power density and efficiency is shown that depends only on the discharging strategy of the flow battery. For a sensible heat source with an inlet temperature of 120 °C and heat transfer at a thermodynamic mean temperature of about 90 °C, the power density reaches 38 W m^(−2) over a complete cycle, and the efficiency reaches 20% of Carnot efficiency. In a benchmarking study, the power production of the flow battery is shown to already achieve 34% of a fully optimized organic Rankine cycle. Thus, TRFBs require further optimization to become a competitive technology for power production and energy storage from low-temperature waste heat.ISSN:2194-4296ISSN:2194-428

Transforming Heat From 90 °C to 110 °C: Demonstration of a Lab-Scale Adsorption Heat Transformer (AdHT)

Industry requires mainly heat above 100 °C, but waste heat is mostly available at temperatures below 100 °C. To close this temperature gap, Adsorption Heat Transformers (AdHTs) are particularly promising and, therefore, recently discussed in the literature. However, an experimental feasibility study of closed AdHTs is currently lacking for temperatures around 100 °C. Therefore, we present an experimental setup for a lab-scale cyclic operating prototype of a closed AdHT in one-bed configuration that uses TiAPSO-34 & H2O as working pair. For the AdHT prototype, we demonstrate that heat transformation from 90 °C to 110 °C is experimentally feasible, while achieving a Coefficient Of Performance (COP) of 0.136 and a Specific Heating Power (SHP) of 32.4 W/kg. Moreover, we identify main challenges of the AdHT cycle that limit the AdHT performance and need to be resolved

Closed-Loop Adsorption-Based Upgrading of Heat from 90 to 110 °C: Experimental Demonstration and Insights for Future Development

Industrial energy efficiency can be increased by recovering waste heat, mainly available below 100 °C. This low-temperature waste heat can drive Adsorption Heat Transformers (AdHTs) to upgrade waste heat to industrially relevant temperatures above 100 °C. Flexible process integration can be achieved by decoupling adsorptive and heat transfer fluid in closed-loop cycles. However, the experimental feasibility of closed-loop AdHTs has not been shown yet. Hence, this work studies an experimental one-bed setup of an AdHT based on a closed-loop cycle using silica gel 123 & water as the working pair. Experimental feasibility is demonstrated for heat transformation from 90 to 110 °C with waste heat released at 25 °C. The highest efficiency COP is 0.183 J J-1 (23% of the maximum Carnot efficiency), and the highest power density SHP is 168 W kg-1. A systematic variation of the operating conditions shows that efficiency and power density strongly depend on the operating temperatures, volume flows, and phase times. Furthermore, avoiding condensation inside the adsorber casing and heat losses are identified to be crucial for the design of AdHTs. In summary, AdHTs based on a closed-loop cycle show a promising performance to recover low-temperature waste heat.ISSN:2194-4296ISSN:2194-428

Water, ethanol, or methanol for adsorption chillers? Model-based performance prediction from Infrared–Large-Temperature-Jump experiments

Adsorption chillers are a promising technology to ease the burden on renewable electricity demand by using waste heat as driving energy. Their performance depends on the adsorbent's kinetics, the chosen refrigerant, the design of the adsorber, and the selected temperatures. Thus, quantifying an adsorption chillers’ performance usually requires expensive experiments at full scale. Here, we efficiently characterize a broad set of working pairs for a wide range of conditions using a two-step approach: First, we conducted Large-Temperature-Jump and equilibrium experiments to extract kinetic and equilibrium parameters of water, ethanol, and methanol with a set of currently discussed adsorbents. Second, we inserted the parameters into a dynamic Modelica model to evaluate the working pairs’ performance in full-scale two-bed adsorption chillers. For chilling above 0 °C, water remains the benchmark refrigerant with the highest volumetric power and efficiency with the silica gels SG123 and Siogel. Ethanol and methanol working pairs offer higher mass transfer coefficients, but could not offset the benefits of water. Coatings of the MOFs aluminum fumarate and CAU–10–H provided exceptionally high heat transfer coefficients, but were too thin to compete. For chilling below 0 °C, ethanol and methanol were on par. ZIF–8 was competitive with the activated carbon CarboTech A35. Overall, CarboTech A35/methanol showed broad applicability, performing comparably to SG123/water above 0 °C while still providing reasonable performance below 0 °C.ISSN:1359-4311ISSN:1873-560

Upgrading Waste Heat from 90 to 110 degrees C: The Potential of Adsorption Heat Transformation

Low-grade heat is abundantly available below 100 degrees C, whereas industry mainly needs heat above 100 degrees C. Thus, the industry cannot directly utilize low-grade heat to save primary energy and emissions. Low-grade heat can be utilized by adsorption heat transformers (AdHTs); however, closed AdHTs to upgrade heat above 100 degrees C are only investigated by idealized steady-state analyses, which indicate the maximal theoretical performance. For evaluating the performance achievable in practice, this work studies a closed AdHT in a one-bed configuration using dynamic simulation. For the working pair AQSOA-Z02/H2O, the performance is optimized via the design of the adsorber heat exchanger and the control of the AdHT cycle. When heat is upgraded from 90 to 110 degrees C, releasing waste heat at 35 degrees C, the maximum exergetic coefficient of performance (COPexergetic) is 0.64, and the maximum specific heating power (SHP) is 590 W kg(-1). The maximum SHP can increase by 35% when releasing waste heat at 25 degrees C. Both performance indicators strongly depend on design, control, and the available temperature of the waste heat. Overall, AdHTs with optimized design and control are promising to utilize low-grade waste heat.ISSN:2194-4288ISSN:2194-429