Space Conditioning Systems Research Articles

Sustainable heating and cooling for residential buildings: coaxial ground heat exchangers

As the world fights climate change challenges and the soaring demand for energy-efficient solutions, the need for sustainable heating and cooling systems in residential buildings has become paramount. Traditional systems rely on fossil fuels and contribute significantly to greenhouse gas emissions. Ground source heat pumps (GSHPs) have emerged as a promising technology, offering a sustainable alternative to conventional space conditioning systems. Central to the success of GSHPs is the efficient and reliable operation of their ground heat exchangers, with lower installation costs, as a prospective solution for future residential applications. This paper presents a numerical axisymmetric (r-z) finite volume method solution of the thermal performance of coaxial ground heat exchangers (CGHE) that incorporate phase change material (PCM) in the ground for GSHP applications. An analysis of the CGHE with PCM grout is presented, highlighting the advantages of heat exchange performance over normal grouts. The temperature comparison on three different points in the grout with different grouting materials was carried out to analyse the behaviour of different grouting materials during the heat exchange process. The investigation from a temperature point of view showed that the inclusion of PCM in the grouting material improves the thermal performance of the CGHE.

Feasibility and investigation of residential HVAC system with combined ground source heat pump and solar thermal collectors in different climates of Iran

Geoexchange is one of the most energy-efficient, environmentally clean, and cost-effective space-conditioning systems available on the market. Seasonal storage of solar energy in geothermal boreholes is sharply interesting as a means of heating and cooling in buildings with different applications. In this study, a closed-loop geo-exchange and solar thermal system with two different heat exchangers are examined in an energy-efficient house. TRNSYS, a simulation software tool, was used to model the yearly performance of the hybrid ground-source heat pump (GSHP) system. Two different types of ground heat exchangers, e.g., tube-in-tube and U-tube were employed. The mass flow rate through the heat exchangers and the solar collector was examined to choose the best scenario for each of them. The economic benefits of a gray water heat recovery system and solar collector were studied; the results show that 517 and 536 US Dollars/Year can be saved from each of them, respectively. System performance analysis was carried out in different cities with various climates in Iran. Moreover, a complete economic assessment was evaluated for all cases. The results show that for the same inlet water temperature to GSHP, the tube-in-tube heat exchanger can pass 45% more flow than the U-tube heat exchanger for the same size as the HP. Furthermore, in the absence of the solar collector, a 2.7 × 108 kJ increase was conducted in the auxiliary heater component yearly. Finally, four different climates were evaluated for the launching of the GSHP system with a solar thermal collector. The hot and dry climate with 27 °C fluid inlet to a heat pump, a total borehole depth of 220 m, and an initial cost of $ 25,047 was selected as the most appropriate weather conditions for launching the system in Iran.

Heat transfer prediction for radiant floor heating/cooling systems using artificial neural network (ANN)

AbstractRadiant floor cooling and heating systems (RHC) are gaining popularity as compared with conventional space conditioning systems. An understanding of the heat transfer capacity of the radiant system is desirable to design a space conditioning system using RHC technology. In the present work, a simplified heat flux model for RHC is developed for both cooling and heating modes of operation. The Artificial Neural Network (ANN) technique is used for the development of the simplified model. Experimental data from literature covering a wide operating range of the RHC is considered for model development and validation. Operating parameters such as mass flow rate (mf), heat resistance (Rs), mean temperature of water flowing through the pipe (Tm), and operative temperature (Top) are considered independent variables influencing the heat flux (qt). The neural network consists of four input layers, one output layer, and one hidden layer with a feed‐forward‐back‐propagation algorithm. A study on the selection of the optimum number of neurons in the range of 1–9 for the hidden layer is also performed. On the basis of the performance parameters, namely, average‐absolute‐relative‐deviation (AARD = 0.11283) percentage, mean‐square‐error (MSE = 0.00055), and the coefficient of determination (R2 = 0.9984), a hidden layer is modeled with five neurons.

Parametric Study of Panel PCM–Air Heat Exchanger Designs

Heat exchangers, devices for the transfer of heat between two or more working fluids, are extensively used in cooling applications and heating applications. Heat exchangers in buildings are typically components of space-conditioning systems, as well as of water-heating applications. Heat exchangers are also sometimes used in applications that require storage and release of energy at specific times. Phase change materials (PCMs) enhance these heat-exchange processes, given their ability to melt and solidify at a fixed range of temperatures, absorbing or releasing significant amounts of latent heat. Five different configurations of PCM–air heat exchangers for thermal control in buildings are analyzed in this work. The heat exchangers were fitted with PCM encapsulated in plastic and composite pouches of various shapes, and packaged in stackable panel layers. Three-dimensional computational fluid dynamics (CFD) modeling of coupled incompressible fluid and conjugate heat transfer were performed on the designs. The phase change process was numerically modelled using the apparent heat capacity method. Steady-state CFD simulations provided quantification of pressure drop as a function of air flow velocity. Transient simulation results describe the thermal evolution of PCM in the pouches, helping to determine the best performing configuration with respect to total thermal charging time.

Functional unit choice in space conditioning life cycle assessment: Review and recommendations

Space cooling through air conditioners and electric fans, which already accounts for 10% of global energy consumption, is expected to increase as temperatures rise under climate change and as air conditioning becomes more accessible. The life cycle greenhouse gas (GHG) emissions of this energy-intensive activity are concerning in their potential to create a positive feedback loop for climate change. There is thus a growing need for life cycle assessments (LCAs) on space conditioning systems to be designed to facilitate comparison for climate change mitigation efforts, with similar standardization as has developed in the electricity LCA field, in which the vast majority of LCAs conform to a specific system boundary and functional unit. We present the first study that assesses the uniformity and identifies discrepancies in methodology between space conditioning LCAs. We analyzed the goals, system boundaries, and chosen functional units of 33 peer-reviewed LCAs on single-structure space conditioning systems published between 2000 and 2020. The findings indicate that the stated goals of these studies were mostly aligned, but there was a lack of consensus on functional unit definition, with multiple different types of units used across studies. This lack of consistency in functional units complicates and may even prevent the comparison of life cycle environmental impacts of technologies and methods of space conditioning, which is necessary to make decisions towards increased sustainability in space conditioning. Consequently, we recommend that a standard functional unit for space conditioning systems be defined as 1 MJ of heat added to or removed from conditioned space to ensure future studies are comparable.

Optimal control of single stage LiBr/water absorption chiller

Low-capacity refrigeration and space conditioning systems have increased significantly in the last years, increasing primary energy demand. For this reason, it is important to start implementing refrigeration and space conditioning system that can be driven by unconventional energy sources, such as a single stage lithium bromide absorption refrigeration chiller since it can be powered by a low grade heat source. Therefore, we establish an optimal control strategy to operate these systems. For the implementation of the optimum control a dynamic model to evaluate the system is developed and discretized and solved using the interior point optimization (IPOPT) solver. The cases studied where a step and a sinusoidal perturbation on the hot water inlet temperature. The results obtained are promising because, through the implementation of the optimal control strategy, the coefficient of performance (COP) of the refrigeration system was significantly improved, reducing operational cost and all this without affecting the cold water outlet temperature.

Cost-effective envelope optimisation for social housing in Brazil's moderate climates zones

Brazil faces a housing deficit of more than 5.5 million units and nearly 11 million existing homes present inadequate living conditions. In response, the Brazilian government launched in 2009 a mass social housing programme with a target of delivering 24 million new units for low-income households by 2022. Their standardised design is relatively cheap to build but does not take into account climatic conditions, thermal comfort and energy efficiency. This could result in poor performance and uncomfortable indoor environments that could impact of the occupier's health and lead to the unnecessary use of energy-hungry space conditioning systems. Given the scale of this programme, the potential impact on Brazil's already debilitated socio-economic and environmental balance is very substantial. In this work, the authors have deployed sensitivity analysis to explore a variety of envelope combinations through dynamic building simulation for three southern Brazilian cities, Curitiba, Sao Paulo and Porto Alegre, aiming to achieve better than the typical performance. Thermal transmittance properties and air permeability rates were tested from the most common construction characteristics in Brazil to some of the highest levels of fabric energy efficiency. These combinations were also analysed in terms of their cost-effectiveness. The results suggested that optimised envelopes could improve thermal comfort by up to 97% in comparison to a typical envelope, but also cost nearly 50% more. Other envelope combinations investigated were shown to be more cost-effective with a significant increase of thermal comfort levels and were therefore considered to be a more adequate solution for the context.

Life-Cycle Assessment of an Innovative Ground-Source Heat Pump System with Upstream Thermal Storage

An innovative space-conditioning system is proposed and a life-cycle assessment (LCA) is presented. The layout is obtained starting from a ground-source heat pump system (GSHP) and includes upstream thermal storage (TS). A prototype of the system, implemented following this new approach, is currently in use for space heating and cooling of an industrial building. As a result of the TS designed to decouple the geothermal side from the heat-pump side, the system is able to provide the required thermal energy to the building with a reduced-size geothermal installation (i.e., shorter/fewer boreholes (BHs)). The performance was monitored for over 2 years, both in cooling and heating modes. A LCA study of this system is performed on the basis of specific data for implementation and operation phases. The results are given in terms of the comprehensive ReCiPe midpoint and endpoint indicator suite and are compared with literature studies of other conventional technologies for space conditioning.

Thermal Conditions Controlled by Thermostats: An Occupational Comfort and Well-being Perspective

From the perspective of occupational comfort and well-being, indoor thermal condition characterized by its temperature levels, spatial variations, and airflow patterns plays an important role. Studies have demonstrated strong correlations among indoor comfort levels and users' well-being, productivity, and overall health. Computational fluid dynamics (CFD) has been used to investigate indoor thermal comfort. In this research, a private office on the University of Cincinnati campus was selected and studied in order to spatially map the thermal comfort index. Autodesk® CFD, a ventilation simulation software, was utilized to model the office space and air conditioning systems, as well as simulate the airflow in the indoor space. Based on the simulation results, the air speed, ambient temperature, and relative humidity were all obtained for different vent locations. These simulated parameters can be used in dynamic anthropometry to acquire the predicted mean vote (PMV) and temperature in specific office areas. Through this method, a visualized indoor comfort map was developed as a means of indicating potential user comfort effects; spatial variations in the indoor comfort index have also been analyzed and discussed.

A-10 空気循環方式による全館空調システム住宅の太陽熱利用に関する研究

A-10 空気循環方式による全館空調システム住宅の太陽熱利用に関する研究

Application of dynamic airflows in buildings and its effects on perceived thermal comfort

In this paper we explore the performance of a newly developed air conditioner which can produce different airflow modes such as dynamically changed simulated natural wind (SNW) and constant mechanical wind (CMW). Climate chamber experiments were conducted to study the demand for air movement and to compare effects of SNW and CMW on human subjects’ thermal comfort perception in warm environment. The results indicate that it was possible for subjects to remain comfortable at 28℃ and 30℃ with a suitable air velocity range of 0.4–0.6 and 0.7–0.9 m/s, respectively. Compared with CMW, SNW produced stronger cooling effect and induced better perceived thermal comfort voting from human subjects. Over 60% of these subjects preferred SNW to CMW. These experimental findings would serve as a reference for the design of more energy saving space conditioning systems in warm climates.

Energy Performance Comparison of Inverter based Variable Refrigerant Flow Unitary AC with Constant Volume Unitary AC

Abstract The Variable Refrigerant Flow (VRF) technology has emerged as promising alternative to conventional systems, especially for small residential and commercial buildings which are largely untouched in regard to the energy efficiency of space conditioning systems despite the huge energy saving potential. This study aims to find out whether VRF split system provides an energy efficient and viable retrofit option for this sector. This paper compares the performance of an inverter based VRF Unitary AC with Constant Volume Unitary AC using the field performance testing. Both systems were installed in two geometrically and thermally identical rooms, with 14.5 m2 floor area each. Observations were made in the months of January to May, the systems were operated from 10 AM to 6 PM, auxiliary heating was also provided to vary the zone internal load conditions; the auxiliary heat load provided by the electrical heaters was 2 kW and 4 kW. The results were analyzed and it was found that the VRF technology is energy saving only when it works on part load conditions; minimum (negative) savings were observed when the outdoor temperature was equal to the rated outdoor DBT i.e. when the system was operating at full load conditions while maximum savings i.e. 40% were observed when the outdoor DBT is nearly equal to the indoor set point temperature.

Characterizing the effect of an off-peak ground pre-cool control strategy on hybrid ground source heat pump systems

Geo-exchange systems are a sustainable alternative to conventional space conditioning systems due to their high operating efficiency, resulting in reduced energy consumption and greenhouse gas emissions. However, geo-exchange’s ability to penetrate the market has been throttled by large capital investments, resulting in undesirable payback periods. Optimized hybrid ground source heat pump systems (HGSHP) systems have been introduced as a remedy to overcome the current economic hurtles associated to the installation of geo-exchange systems. In both the literature, as well as in practice, there still remains potential for increased economic feasibility of this technology through integration of intelligent operational strategies. This paper presents a novel control methodology referred to as an off-peak ground pre-cool strategy, employing a time-of-use conscious operating logic which artificially pre-condition the system’s bore-field. Reducing peak power consumption is achieved by creating improved thermal characteristics during mid-peak/peak time-of-use operating brackets. A comprehensive numerical model was developed to characterize the operation of HGSHP systems for three real case studies. The model implemented a base case set-point control scheme, used as a reference to assess the operational benefit of the proposed off-peak ground pre-cool control strategy. The preliminary analyses indicated operational cost savings of up to 16.4%, under specific pre-cool scheduling. The strategy indicated reductions in both carbon emission and peak power consumption of up to 15.0% and 58.5%, respectively. In all cases increasing cooling supplied by the hybrid geo-exchange system was indicated, with a maximum observed capacity increase of 43.7%.

The underlying linkage between personal control and thermal comfort: Psychological or physical effects?

In this paper we explore the influence of personal control over space conditioning systems on occupants’ thermal comfort perception. It aims to identify the psychological effects of perceived control as well as the physical impacts when the controllable approaches are utilized. A chamber experiment was conducted, the results from which indicate that subjects with perceived control tended to report more positive comfort perception. The severer the thermal conditions were, the more subjects wanted to use personal control approaches, thus corresponding to stronger psychological effects. When personal control approaches were utilized appropriately, even a slight improvement in thermal conditions could significantly alleviate subjects’ thermal discomfort complaints. These findings provide support to the adaptive comfort theory and can serve as reference for the design of personal environmental control systems.

Experimental assessment of a compact branched tray generator for ammonia–water desorption

An experimental investigation of heat and mass transfer in a miniaturized ammonia–water desorber utilizing microscale geometries is presented. The desorber, sized for a 3.5-kW space-conditioning system, is of a branched tray design consisting of an array of alternating plates with integral microscale features enclosed between cover plates. The desorber is hydronically coupled to a heat transfer fluid that is heated by the combustion of natural gas. An adiabatic analyzer and solution cooled rectifier are integrated into the same envelope as the desorber. The component is tested as part of a single-pressure system on a breadboard test facility and is studied over a range of heat transfer fluid inlet temperatures, flow rates, and concentrated solution flow rates. Desorber performance is experimentally investigated over a wide range of test conditions to improve the understanding of performance at design and off-design conditions and the potential for flow instabilities.

Numerical Investigation of Nanofluid-based Solar Collectors

Solar thermal collectors are applicable in the water heating or space conditioning systems. Due to the low efficiency of the conventional collectors, some suggestions have been presented for improvement in the collector efficiency. Adding nanoparticles to the working fluid in direct absorption solar collector, which has been recently proposed, leads to improvement in the working fluid thermal and optical properties such as thermal conductivity and absorption coefficient. This results certainly in collector efficiency enhancement. In this paper, the radiative transfer and energy equations are numerically solved. Due to laminar and fully developed flow in the collector, the velocity profile is assumed to be parabolic. As can be observed from the results, outlet temperature of collector is lower than that obtained using uniform velocity profile. Furthermore, a suspension of carbon nanohorns in the water is used as the working fluid in the model and its effect on the collector efficiency is investigated. It was found that the presence of carbon nanohorns increases the collector efficiency by about 17% compared to a conventional flat-plate collector. In comparison with the mixture of water and aluminium nanoparticles, a quite similar efficiency is obtained using very lower concentration of carbon nanohorns in the water.

Thermal load mitigation and passive cooling in residential attics containing PCM-enhanced insulations

Residential attics has the potential to be one of the most energy efficient building components by combining thermal processes of attic floor insulation, attic air space, ventilation in attics, and solar collecting roof decks. Large amounts of solar energy collected by the roofs in cooling-dominated and mixed climates generate excess cooling loads, which need to be removed from the building by the space conditioning systems. This paper investigates potential ways to improve the thermal design of the residential home attics to minimize the cooling energy consumption in the cooling-dominated and mixed climates. Dynamic thermal characteristics of thick attic floor insulations and blends of phase change materials (PCMs) with insulations are analyzed. Both approaches can provide notable reductions of thermal loads at the attic level. In addition, a significant time shift of peak-hour loads can move a major operation time for air conditioning system from the daytime peak hours to nighttime low demand hours. A reverse heat flow direction can be observed during the day in the case of really thick layers of bulk insulation or PCM-enhanced insulations, compared to the rest of the building envelope components. This effect may provide free passive cooling to the building, and can be very useful in locations of double electrical tariffs with high daytime peak-hour electric energy rates and less-expensive off-peak energy cost. In both of the above cases, an addition of PCM to the bulk insulation brings substantial performance enhancement not available for traditional insulation applications. This paper presents a short overview of dynamic material characteristics and energy performance data necessary for future dynamic applications of different configurations of the attic floor insulation and PCM-insulation blends in residential homes. A series of whole-building scale and material scale numerical simulations were performed on a single story ranch house to analyze potential energy savings and optimize location of PCM within the attic insulation.

Falling-Film Absorption Around Microchannel Tube Banks

An experimental investigation of heat and mass transfer in a falling-film absorber with microchannel tube arrays was conducted. Liquid ammonia–water solution flows in a falling-film mode around an array of small diameter coolant tubes, while vapor flows upward through the tube array counter-current to the falling film. This absorber was installed in a test facility consisting of all components of a functional single-effect absorption chiller, including a desorber, rectifier, condenser, evaporator, solution heat exchanger, and refrigerant precooler, to obtain realistic operating conditions at the absorber and to account for the influence of the other components in the system. Unlike studies in the literature on bench-top, single-component, single-pressure test stands, here the experiments were conducted on the absorber at vapor, solution, and coupling fluid conditions representative of space-conditioning systems in the heating and cooling modes. Absorption measurements were taken over a wide range of solution flow rates, concentrations, and coupling fluid temperatures, which simulated operation of thermally activated absorption systems at different cooling capacities and ambient conditions. These measurements are used to interpret the effects of solution and vapor flow rates, concentrations, and coupling fluid conditions on the respective heat and mass transfer coefficients.

Sunlight absorbing potential of carbon nanoball water and ethylene glycol-based nanofluids

Solar thermal collectors are applicable in the water heating or space conditioning systems in which surface-based absorption of incident solar flux cause high heat losses. Therefore, an enhancement in the efficiency of solar harvesting devices is a basic challenge which requires great effort. Adding nanoparticles to the working fluid in direct absorption solar collector, which has been recently proposed, leads to improvement in the working fluid thermal and optical properties such as thermal conductivity and absorption coefficient. This results certainly in collector efficiency enhancement. In this paper, the characteristics of nanofluids consisting carbon nanoball in water- and ethylene glycol-based suspensions in consideration of their use as sunlight absorber fluid in a DASC are investigated. It was found that by using of 300 ppm carbon nanoballs, the extinction coefficient of pure water and ethylene glycol is increased by about 3.9 cm−1 and 3.4 cm−1 in average, respectively. With these significantly promising optical properties, a direct absorption solar collector using carbon nanoball-based nanofluids can achieve relatively higher efficiencies, compared with a conventional solar collector.

Multi-structural fast nonlinear model-based predictive control of a hydronic heating system

Buildings consume a significant amount of energy to maintain the indoor thermal comfort. One way to reduce the energy consumption in buildings is to improve the overall energy efficiency through integrated advanced controls. It is undoubted that incorporating a model and utilizing future information in real-time building operation offers great energy saving potentials. However, there are many barriers preventing this from happening. Building systems are nonlinear and multiple-input-multiple-output (MIMO) in nature. Conventional approach with a global search solver and a detailed model simulator or direct nonlinear model predictive control (MPC) incurs prohibitive computational cost. This paper proposes a multi-structural fast nonlinear model predictive control (MPC) for handling nonlinear building systems and applied it on a hydronic heating system. The methodology remains the advantages of linear classical MPC and solves the nonlinearity issues involved in thermal comfort and energy conservation oriented control. The simulation shows that the controllers can achieve the optimal solutions in less than five minutes for five days simulation in a full look-ahead scenario with a Duo T6400 2.0 GHz computer. The fastest scenario takes only thirty more seconds to accomplish, which makes the approach feasible for online implementation. The techniques of using MPC for achieving smooth day-night switch, band control, dynamic constraints, and dynamic weighting are discussed. The energy saving potential of applying the proposed MPCs is found to be between six to forty two percent. In addition, the techniques decouple the building system from the mechanical system and are applicable to other space conditioning systems as well.