Passive Heat Recovery Research Articles

A review of recent studies of both heat pipe and evaporative cooling in passive heat recovery

A review of recent studies of both heat pipe and evaporative cooling in passive heat recovery

Performance of a BAPVT modules coupled TEHR unit fresh air system based on micro heat pipe array

Energy consumption for heating and cooling fresh air has become a major component of building energy consumption, and there is a growing focus on highly-efficient and energy-conserving fresh-air treatment technology. This paper presents a new fresh-air system based on a high-efficiency heat transfer element, the micro heat pipe array (MHPA). It consists of a novel air-cooled building attached to a photovoltaic/thermal module (MHPA-BAPVT) and a thermoelectric heat recovery unit (MHPA-TEHR). The system uses solar energy, photovoltaic power generation, and waste heat for fresh air preheating. A series of fresh-air systems were designed, constructed, tested, and analyzed. The results reveal that on sunny winter days, the temperature of fresh air flowing through MHPA-BAPVT modules can increase by 22.15 °C. The average thermal and electric efficiency of the MHPA-BAPVT modules was 12.16% and 7.10%, respectively, and MHPA-TEHR could exceed the upper limit of passive heat recovery technology, which could generate 1.25 kW of heat per 1 kW of input electric power. The maximum ratio of total heating power to the total power consumption of the fresh-air system reached 14.94. During a typical day in the winter season, solar preheating exceeded the fresh air heat requirement by 24.34%, and the excess heat provided 292.92 MJ for building heating demand. Comparing the electric heating fresh-air unit with conventional heat recovery, the former achieved nearly zero energy consumption during the winter season. During the transitional season, the system could provide 711.82 MJ of solar energy, which improved the indoor temperature. Finally, using the experimental data and meteorological parameters of Zibo, the annual heat and electricity benefits and reduced carbon dioxide emissions of the fresh-air system were calculated. The fresh-air system in this study can fully utilize solar energy for heat collection and power generation to reduce building energy consumption and can be applied widely.

Passive heat recovery wind tower: Assessing the overheating risk in summertime and ventilation heat loss reduction in wintertime

Wind towers are low-energy ventilation devices which can provide cooling and reduce buildings’ energy use. However, during unfavourable climate conditions, wind tower operation could cause thermal discomfort and ventilation heat loss. There also has been very limited research into the overheating risk caused by wind tower ventilation. Therefore, this study explores the feasibility of the year-round operation of wind towers with solid tube heat recovery (THR) through computational fluid dynamics (CFD). The results show that within the mild-cold months, the natural ventilation wind tower with THR raised the room temperature by an average of 3.1 °C, based on the set conditions. This extended the working period of the wind tower throughout the year, especially during mild-cold months. During summertime, the highest indoor temperature observed in the space ventilated by the wind tower with THR was 24.35 °C, which meets the static overheating criteria. The wind tower with THR reduced the ventilation heat loss by up to 8.1% in wintertime. It should be acknowledged that the wind tower with THR cannot provide satisfactory thermal comfort in cold months with outdoor temperatures below 9.41 °C, and more research should be conducted to improve the heat recovery efficiency.

Entropy generation minimization of an advanced two-bed adsorption refrigeration system

This article presents the thermodynamic assessment of an advanced adsorption chiller with an aim towards entropy generation minimization through the selection of appropriate operating strategies, temperatures, and design modifications. The present study carries out a second law analysis of a two-bed silica gel water pair-based adsorption cooling system, with a focus on the under-explored aspects of a heat recovery strategy and auxiliary consumption-reducing measures. A second law performance index, viz. specific irreversibility, is introduced for effectively incorporating the entropy generation, auxiliary electricity consumption, and cooling capacity, and it has been further verified to be an indicator of second law efficiency. The performance of the system has been numerically evaluated under the normal and passive heat recovery strategies, where it is found that the passive heat recovery strategy offers a lower entropy generation by 63%. The study further investigates the second law efficiency impact of adopting a capillary-assisted evaporator for auxiliary electricity consumption reduction. It is observed that the specific irreversibility could reduce by up to 22% over a conventional falling film design. The analysis and results presented in this study are anticipated to increase the effectiveness of adsorption chillers for heat recovery applications.

Wind tunnel test and numerical study of a multi-sided wind tower with horizontal heat pipes

Passive ventilation such as wind towers can ventilate spaces by using regional pressure differences and the stack effect. Wind towers are often closed throughout the winter to prevent an increase in heating energy demand. Heat pipes can be installed to transfer heat from the outgoing to the incoming air, however the inclusion of heat recovery can incur a pressure drop that negatively impacts ventilation rates. Here it is shown that with an array of horizontally arranged heat pipes, ventilation rates of 0.1 m3/s are maintained at a 1 m/s inlet velocity. The incoming air temperature was raised by up to 2.8 °C, thereby increasing the operational window of the passive ventilation system. By mounting the heat pipes horizontally through the wind tower, direct heat transfer is facilitated between the inlet and outlet. The results demonstrate how a passive ventilation and heat recovery system is likely to operate in the winter months of a cooler climate according to the wind speed and temperature difference between the fresh and exhaust air. It is intended that this system will be further developed to provide both heating and cooling in the winter and summer respectively by installing a seasonal thermal loop.

Passive Heat Recovery Wind Tower: Assessing the Overheating Risk in Summertime and Ventilation Heat Loss Reduction in Wintertime

Passive Heat Recovery Wind Tower: Assessing the Overheating Risk in Summertime and Ventilation Heat Loss Reduction in Wintertime

Numerical investigation on turbulent flow and heat transfer characteristics of ferro-nanofluid flowing in dimpled tube under magnetic field effect

• This is the first study on dimpled tube under magnetic field effect. • Effects of dimple geometry and magnetic field on the thermal and hydrodynamic behaviour have been investigated. • The Nu enhances with increasing volume fraction of magnetic nanoparticle. • The highest PEC is obtained for Ha = 75, magnetic nanoparticle fraction of 2.5 vol% and dimpled tube with P 2 /d = 7.5. For the aim of increasing the heat transfer enhancement, a hybrid method in which active and passive heat recovery techniques have been used together. The usage of nanofluid, MHD and dimpled fins tube have not been utilized together so far. Regarding this issue, this study is the first numerical study to determine effect of usage of three effects together comprehensively. In this study, thermo-hydraulic performance of Fe 3 O 4 /H 2 O nanofluid (ferro-nanofluid) flow inside dimpled tube under magnetic field effect has been examined numerically. The main purpose of the study is to obtain numerical data for turbulent flow in the spherical dimpled tubes providing some aid to design a highly efficient thermal energy storage devices. Dimple geometry with nondimensional pitch ratio ( P/d = 3.75, 7.50 and 11.25 ), Hartmann number ( Ha = 75, 150, 225) and nanoparticle volume fraction ( φ = 0.5, 1.0 and 2.5 vol % ) are the parameters investigated in this study. The numerical analyses have been carried out Reynolds number ranging from 10,000 to 50,000 at a constant heat flux at 20 kW/m 2 . The simulations have been built up by Realizable k- ε turbulence model and single-phase approach. Also, “MagnetoHydroDynamic” ( MHD ) module has also been activated for defining magnetic field effect. The results showed that Nusselt number increases with increasing Reynolds number and decreasing pitch ratio. The dimple geometry type of P/d = 7.50 has been determined as the most efficient dimple geometry type. In the case of highest magnetic field intensity, the highest Nusselt number increment (72.48%) has been obtained for φ = 2.5 vol% compared to the base fluid of distilled water using as the working fluid for smooth tube. The highest PEC value was also obtained as 1.126 for the case of P/d = 7.5, φ = 2.5 vol% and Ha = 75. In addition, the effect of magnetic field intensity on velocity and temperature distributions has been presented with contour graphs.

CFD investigation of a natural ventilation wind tower system with solid tube banks heat recovery for mild-cold climate

Passive strategies are increasingly being employed in buildings to minimize their total energy consumption and emissions. Wind tower, a natural ventilation device, can capture the wind flow from higher locations and supply the air into the buildings' indoor space without consuming electric power. However, its use in mild-cold climate is limited due to excessive heat loss and thermal discomfort, in particular, during winter. Limited research has been conducted to explore the pre-heating of the supply air in the wind tower to address the issue and potentially increase its adoption in mild-cold climates. Therefore, a novel wind tower ventilation system integrated with solid tube banks heat recovery (HR) is proposed in this research. By combining with a passive heat recovery device, a wind tower can be operated under winter conditions to improve indoor ventilation while reducing the building heating energy consumption. A three-dimensional computational fluid dynamics (CFD) model was developed to investigate the effects of the longitudinal pitch (SL) and the transverse pitch (ST) of the HR device on the ventilation and thermal performance. The overall performance of the wind tower was evaluated under different outdoor wind speeds. The results show that the heat recovery can be improved by reducing SL and ST, raising the supply fresh air temperature by up to 6.4 °C. Furthermore, the proposed wind tower can provide sufficient ventilation for a typical classroom occupied by 15 people when the external wind speed exceeds 3 m/s.

Climate change resilience of school premises in Cyprus: An examination of retrofit approaches and their implications on thermal and energy performance

Educational buildings form a significant part of non-residential buildings that are of public use, and, in the case of Cyprus, this building stock tends to be less energy-efficient compared to a number of other public building typologies. Moreover, it is the purpose, frequency of use and user density of these buildings that make them a special case for study. These buildings are no longer fit for their assigned purpose as they require significant energy for heating in the winter and cooling in the summer, to avoid overheating. These conditions cause significant concerns in school buildings in Cyprus and the effects of climate change exacerbates these parameters. This research aims to evaluate the effectiveness of different retrofitting measures and their impact on the thermal comfort and the energy performance of educational premises under different climatic scenarios. For the purposes of this study, a secondary school building was selected as a representative case study of educational buildings in Cyprus and was modelled using IES-VE. Retrofit measures optimizing geometry, construction and operation were examined using a calibrated dynamic thermal simulation model in current and future climatic scenarios, to determine both their effectiveness in minimizing heating demand in the winter and overheating conditions in the summer period. Comfort conditions were assessed with reference to international standards based on adaptive thermal comfort. The study demonstrates that most educational buildings in Cyprus cannot ensure thermal comfort conditions, without the support of technical systems, neither in the present nor in anticipated future climatic conditions, thereby indicating the urgent need to take action. According to the research results, natural ventilation and insulation of the roof are considered very important in achieving this goal, as they have the potential to minimize the cooling degree hours by 96.8% and the heating degree hours by 4.3%. A combination of all passive measures and heat recovery ventilation can reduce degree hours for both heating and cooling by 51% in the current climatic conditions, and by 60% and 66.4% according to climatic projections for 2050 and 2090, respectively. The methodology for the evaluation of the thermal and energy behaviour of the educational building stock applied in this study could be a trustworthy tool for decision-making in energy retrofitting projects in Cyprus, as well as in other areas with similar climatic conditions.

Operational envelope and performance enhancement of a two-bed adsorption cooling system

Along with the measures to improve the design of the adsorption cooling systems, the operating strategy selection plays an equally important role in their performance enhancement. Though several strategies such as heat recovery, mass recovery, multi-staging, etc., have been reported in the literature, their implementation in the more popular small-scale two-bed systems has not been given due attention. The present study highlights the performance enhancement of a two-bed single-stage silica gel-water adsorption cooling system through novel interventions in the operating cycle. A facility has been developed for testing a 10 kW water-cooled adsorption chiller. Passive heat recovery and two-stage operational strategies have been proposed for the enhancement of performance and operational envelopes through modifications in the chiller’s valves sequencing. An improvement of 23–42% in coefficient of performance (COP) is observed with an optimized passive heat recovery strategy, while maintaining the water level in the hot water tank. Two kinds of two-stage operations viz., conventional two-stage and reheat two-stage, show potential for performance improvement under extreme conditions, resulting in an operational envelope enhancement up to 56%. A theoretical approach based on the thermodynamic analysis of adsorbent limits has been presented to assess the maximum desorption temperature limits.

Effects of passive heat stress and recovery on human cognitive function: An ERP study.

Using event-related potentials (ERPs), we investigated the effects of passive heat stress and recovery on the human cognitive function with Flanker tasks, involving congruent and incongruent stimuli. We hypothesized that modulation of the peak amplitude and latency of the P300 component in ERP waveforms would differ with task difficulty during passive heat stress and recovery. Subjects performed the Flanker tasks before (Pre), at the end of whole body heating (Heat: internal temperature increase of ~1.2°C from the pre-heat baseline), and after the internal temperature had returned to the pre-heat baseline (Recovery). The internal temperature was regulated by a tube-lined suit by perfusing 50°C water for heat stress and 25°C water for recovery immediately after the heat stress. Regardless of task difficulty, the reaction time (RT) was shortened during Heat rather than Pre and Recovery, and standard deviations of RT (i.e., response variability) were significantly smaller during Heat than Pre. However, the peak amplitudes of the P300 component in ERPs, which involved selective attention, expectancy, and memory updating, were significantly smaller during Heat than during Pre, suggesting the impairment of neural activity in cognitive function. Notably, the peak amplitudes of the P300 component were higher during Recovery than during Heat, indicating that the impaired neural activity had recovered after sufficient whole-body cooling. An indicator of the stimulus classification/evaluation time (peak latency of P300) and the RT were shortened during Heat stress, but such shortening was not noted after whole-body cooling. These results suggest that hyperthermia affects the human cognitive function, reflected by the peak amplitude and latency of the P300 component in ERPs during the Flanker tasks, but sufficient treatment such as whole-body cooling performed in this study can recover those functions.

Prototype of an air to air thermoelectric heat pump integrated with a double flux mechanical ventilation system for passive houses

Abstract This paper describes the design of an air-to-air thermoelectric heat pump for its integration with a double flux mechanical ventilation system for domestic use in Passive House standard. The prototype has been built and thermally characterized in a test bench reproducing winter and summer conditions, with different gaps between indoor and outdoor temperatures. In addition, two different integration possibilities have been analyzed and tested: a stand-alone installation and the combination with a heat recovery unit. This prototype is composed of 10 thermoelectric modules and finned heat pipes to transfer the heat between the modules and the incoming and outgoing ventilation flows. The maximum heating capacity with 12 V supply was proven to be 1,250 W for heating and 375 W for cooling, with COPs ranging 1.5–4 and 0.5–2.5 respectively. Results show the variations in the performance of the thermoelectric heat pump depending on the voltage supply (3–12 V), the air flows (55–130 m3/h) and the temperature gaps between them. This paper demonstrates the convenience of combining passive and active heat recovery technologies (thermoelectric pump coupled to a heat recovery unit), bringing improvements on the thermal power higher than 25% for heating and 10% for cooling, with respect to the thermoelectric heat pump working directly between the incoming and outgoing air flows. The COP is also increased, especially for low energy demands, when the voltage is 3–6 V. In these cases, the COP might be improved by 50% for heating and 30% for cooling.

Development of a natural ventilation windcatcher with passive heat recovery wheel for mild-cold climates: CFD and experimental analysis

Abstract User demand for increased internal thermal comfort conditions have resulted in rising energy costs for space-heating consumption. The present study aims to recover the thermal energy in ventilation exhaust air and transfer the energy to the incoming air, to be redistributed using natural ventilation windcatcher. A comprehensive review was carried out to explore heat recovery systems that can potentially be incorporated with natural ventilation wind catchers. A rotary heat recovery device suitable to be incorporated with a roof mounted multi directional windcatcher system was developed. Computational Fluid Dynamics (CFD) modelling and laboratory experimental tests were conducted to investigate the proposed system. In the first phase, a full-scale prototype of the passive rotary thermal wheel device was developed and tested in a crossflow channel to initially assess the concept and performance of the design. Two configurations of the passive heat recovery wheel were tested: 20 and 32 radial blades. The second phase focused on investigating the integration of heat recovery wheel into a windcatcher system. CFD modelling and scaled wind tunnel testing were conducted to assess the airflow and temperature distribution around the multi-directional windcatcher with a passive rotary wheel. The results showed that the addition of the heat recovery wheel rotating at 15 rpm reduced the indoor airflow speed between 14 and 30%, depending on the outdoor wind conditions. The system was able to provide the recommended fresh air rates when the outdoor wind speed was 1.5 m/s and higher. In addition to sufficient ventilation, the heat recovery system had a positive impact on the indoor air temperature, raising the temperature up to 3.7 °C depending on the indoor/outdoor conditions.

Analysis of a rotary passive heat recovery device for natural ventilation windcatcher

Based on the design of traditional architecture, windcatchers are devices which provides passive ventilation by manipulating pressure differentials around buildings induced by the movement of wind and difference in temperature. In temperature climates, it is effective in providing passive cooling during summer months. However, during winter months, the low air temperature supplied to the space can cause further thermal discomfort and increase heat loss which lead to higher energy consumption. This limits the capabilities of windcatchers to provide ventilation all year round. To address this issue, the present study proposes incorporating a rotary thermal heat recovery device into the windcatcher channel and investigate its performance using numerical modelling and experimental tests. The study focused on characterizing the design and performance of the copper radial blades of the proposed heat recovery device. The predicted results of the airflow and temperature showed good agreement with the experimental tests. Two types of radial blade designs were assessed in terms of the airflow velocity and distribution, the pressure drop and the heat recovery performance.

Numerical and experimental analysis of a natural ventilation windcatcher with passive heat recovery for mild-cold climates

In this work, a novel design incorporating a passive heat recovery device into a windcatcher was proposed and investigated using numerical and experimental analysis. The proposed system incorporates a rotary thermal heat recovery in the windcatcher channel. Computational Fluid Dynamics (CFD) was used to investigate the effect of the heat recovery device on the performance of the windcatcher, highlighting the capabilities of the system to deliver the required fresh air rates. The windcatcher model was incorporated to a 5m x 5m x 3m test room model representing a small classroom. The study employed the CFD code Fluent18 with the k-epsilon model to conduct the simulations. The numerical model provided detailed analysis of the airflow and temperature distribution inside the test room. A 1:10 scale prototype of the system was created and tested experimentally in a closed-loop subsonic wind tunnel to validate the CFD investigations. Despite the blockage of the rotary t heat recovery wheel, ventilation rates were able to provide adequate ventilation. In addition to sufficient ventilation, the heat in the exhaust airstreams was captured and transferred to the incoming airstream, raising the temperature between 1-4K depending on the indoor/outdoor conditions, this passive recovery has the potential to reduce demand on space heating systems. According to WBCSD, a recovery of 3 K from the exhaust stream to the inlet stream could generate energy savings up to 20% in heating costs. This shows that the concept has significant potential to be developed further, whereby the heat transfer properties of the system can be investigated and tested on a larger scale.

Experimental study and performance analysis of solar-driven exhaust air thermoelectric heat pump recovery system

Exhaust air heat recovery is of great significance for building energy conservation. Since passive heat recovery systems use temperature or enthalpy difference between outdoor air and indoor air to drive the system, the temperature of fresh air supply cannot meet indoor requirements and the exhaust heat is not fully recovered. In this study, a solar-driven exhaust air thermoelectric heat pump recovery (SDEATHP) system is tested and evaluated for its ability to recover thermal energy from exhaust air to cool or heat fresh air. An experimental platform was established to test its performance. Results show that the SDEATHP system can obtain higher fresh air supply temperature in winter and lower fresh air supply temperature in summer. The system requires only 3.12 W of power for the fans, and the average relative cooling coefficient in summer and the average relative heating coefficient can reach 50.6 and 57.9, respectively. The optimal operating current and voltage of TE modules and photovoltaic system is analyzed, and then the number and types of electrical connections for the TE modules in SDEATHP system are discussed. The SDEATHP system provides a new method for building energy recovery and fresh air supply.

Experimental characterization of an n-pentane oscillating heat pipe for waste heat recovery in ventilation systems

A tubular oscillating heat pipe (OHP), with long form factor and 1 × 0.5 m2 footprint, was experimentally characterized for passive waste heat recovery via air-to-air heat exchange in a typical Heating Ventilation & Air Conditioning (HVAC) ducting system. Experiments were designed to demonstrate the OHP's ability to utilize otherwise wasted thermal energy to pre-heat or pre-cool air for reducing building energy consumption. Results indicate that the OHP is fully-capable of operating while possessing a relatively long form factor (>1 m) and as a forced convection, air-to-air heat exchanger for waste heat recovery in HVAC systems.

A REVIEW OF WASTE HEAT RECOVERY BY USING THE HEAT PIPE AND THERMO-ELECTRIC GENERATOR

This paper continued a detail review of heat pipe and thermoelectric generator application in industrial, automobile and solar waste heat recovery. Each day new greenhouse gasemission further accelerates global warming. The exhaust heat recovery system by usingheat pipe and TEGsis topotentially reduce the greenhouse gases emission. The use of two specific technologies heat pipe and TEGs make passivesolid state system design. Heat pipe used in system consents more flexible design as TEG location not inadequate to exhaust pipe surface. The conversion efficiency of thermoelectric generator is function of temperature difference Heat source and heat sink temperature. The thermal performance of thermoelectric generator will be improved or enhanced by using heat pipe as a heat source in thermoelectric generator. Heat pipes do have limitations such as maximum rates of heat transfer and temperature limits. TEGs do have limitations such as low temperature limits and relatively low efficiency. When used in conjunction, these technologies have the potential to create a completely solid state and passive waste heat recovery system.

Thermal and Optical Analysis of a Passive Heat Recovery and Storage System for Greenhouse Skin

The thermal performance of a greenhouse can be greatly affected by the thermal and optical properties of its envelope system. In this study, a novel skin for greenhouse consisting of ethene-co-tetrafluoroethene (ETFE) membrane and Phase Change Material (PCM) RT28 has been developed and has also been experimentally investigated. The optical behaviour of the developed ETFE-Phase Change Material module sample is measured using a spectrometer and a pyranometer, respectively. The results show that at liquid state, the module has higher transmittance than that of at solid state. In addition, the light transmittance is related to the PCM's temperature. In the thermal aspect, the ETFE-Phase Change Material module presents different characterisation under various irradiances. Comparative analysis is also conducted for the ETFE-Phase Change Material, ETFE-water and ETFE alone. The ETFE-Phase Change Material system shows a benefit of the thermal management than that of other systems.

Passive Heat Recovery System Using Combination of Heat Pipe and Thermoelectric Generator

A different ways of waste heat recovery and conversion to electricity using a thermoelectric generator (TEG) assisted by heat pipes is introduced in this paper. The system consists of the TEG sandwiched between two heat pipes, one connected to the hot side of the TEG and the second one connected to the cold side of TEG. Experiments were conducted for a proof of concept of such system, and the experimental result was used to validate the theoretical model developed. It was found that both the experimental and the theoretical results share the same graphical trend in term of the rate of heat transfer and the effectiveness of heat exchanger. Further investigation in the theoretical model can narrow down the error.