Search

In this paper, we introduce a real-time modelling approach to predict the heating and cooling energy consumption of each housing unit in multi-family residential buildings. We first present measure...

In the U.S., with about 64% of primary occupied homes having unitary air conditioners, a small improvement in the energy efficiency of residential cooling equipment can lead to significant energy savings. Currently, standardized equipment energy efficiency rating is based on standard AHRI 210/240, that provides a metric for comparing the performance of different equipment. However, it is generally recognized that this approach fails to appropriately rate and credit equipment with advanced controls and variable-speed components. Contrarily, a load-based testing and rating approach is presented in this paper that can capture equipment performance with its integrated controls that is more representative of the field. In this approach, representative building sensible and latent loads are emulated in a psychrometric test facility at different indoor and outdoor test conditions utilizing a virtual building model. The indoor test room conditions are continuously adjusted to emulate the dynamic response of the virtual building to the test equipment sensible and latent cooling rates and the equipment dynamic response is measured. Moreover, an automated testing procedure is presented along with convergence criteria for measuring equipment steady-periodic performance. Climate-specific cooling seasonal performance ratings can be determined by propagating the coefficient of performance (COP) from load-based testing results through a temperature-bin method to estimate a seasonal COP (SCOP). In this work, the proposed approach was implemented to test and rate a variable-speed residential heat pump. Furthermore, the sensitivity of load-based testing results to virtual building parameters was investigated. Finally, repeatability results from the load-based testing approach are presented.

Using a vapor compression cycle for cooling in microgravity environments was already suggested in the 1970s to leverage the high coefficient of performance. Since then, only a few systems have operated in microgravity with scarce documentation of these flights. The lack of measured data and detailed documentation makes identifying the necessary next steps difficult for researchers entering the field of refrigeration in space. This paper provides a review of available literature for vapor compression systems flown in microgravity by outlining the history of vapor compression devices in space and presenting performance data. Moreover, gaps in the literature are highlighted and open questions are posed based on the reviewed material. Next steps of research are suggested to support and ultimately achieve reliable vapor compression refrigeration in space. Calculating equivalent masses for a fair comparison of different microgravity cooling technologies is proposed by capturing both energy consumption and used volume.

Previous mode switching algorithms for heat exchanger moving boundary models in the literature are composed of a set of IF-THEN rules. These representations could lead to numerical challenges due to the inherited discontinuities associated with IF-THEN rules. This paper presents an alternative mode switching methodology which results in a continuous moving boundary heat exchanger model over all possible mode changes. Numerical performance of the proposed method for a heat exchanger has been tested using simulations and sample results are compared with a moving boundary model with switching based on IF-THEN rules and also a finite-volume method. The proposed switching algorithm was implemented within a complete vapor compression cycle model and results were compared with experimental data for a start-up transient.

In comparison with other types of conventional positive displacement compressors for domestic refrigerators, oil-free linear compressors have a number of advantages due to their compactness, low friction, limited number of moving parts, and excellent performance. Limited work was found in the open literature related to comprehensive compressor modeling and model validation of commercially available linear compressors. A comprehensive and generalized simulation model of linear compressors has been recently presented by the same authors. The current paper expands on this work by using experimental data for the simulated compressors to characterize their performance over the operating envelopes and to validate the linear compressor model. The validated model was then used to quantify the major sources of losses and analyze system vibration as well as piston eccentricity, which ultimately could be used to design the next generation of linear compressors.

Recently, linear compressor technology has gained increased attention in vapor compression systems due to its compactness and lower frictional losses compared with the conventional reciprocating compressors. Since the absence of the crank-mechanism eliminates the rotation and reduces the side loads on the piston centered inside the cylinder, it is possible to operate the linear compressors without oil lubrication. A number of advantages can be obtained in terms of the refrigerant compatibility, operation conditions and the system cost. In order to enable oil-free operation, while minimizing frictional losses and leakage flow between the piston and the cylinder wall, a gas bearing approach is applied. Little work was found in the open literature related to the numerical analysis of gas bearings coupled with a comprehensive dynamic linear compressor model. This paper presents a 1-D gas bearing model based on a Finite Volume Method (FVM) applied to a linear compressor. The dynamic characteristics of the gas bearings on the gas film pressure field are analyzed using the model. When integrated together with a comprehensive dynamic linear compressor model, it is possible to predict leakage and frictional losses within the compressor as well as the overall performance.

This paper presents a near-optimal control algorithm for minimizing energy costs in hybrid cooling plants in response to cooling loads. The approach is based upon an approximate solution to the par...

Developing a computationally efficient but accurate building energy simulation (BES) model is important for many purposes. Model order reduction (MOR) methods are attractive and much more reliable ...

This paper presents the application of a multi-agent control methodology to large chiller plants. The approach, originally conceived to automate controller design and reduce engineering costs in bu...

Building audits are conducted in many commercial buildings to identify opportunities to reduce energy costs and improve building operation. Because audits require significant effort by building eng...

In this paper, the first open-source generalized simulation framework for positive displacement machines (PDSim) reported in literature is employed to model different compressor and expander types. In particular, it is shown how PDSim handles specific numerical and modeling aspects common to positive displacement machines such as stiffness of differential equations, discontinuous volume curves, leakage flow models, in-chamber and shell heat transfer models, mechanical and frictional losses. A control volume approach based on a set of differential equations is used to predict the performance of different positive displacement machines under steady-periodic operations. Reciprocating, scroll, and two rotary compressors as well as a single-screw expander have been considered as examples. The source code of PDSim with examples are provided as an electronic annex.

A novel generalized framework is presented that can be used to simulate the quasi-steady-state performance of a wide range of positive displacement compressors and expanders (scroll, piston, screw, rotary, spool, etc.). The complete simulation algorithm is described, and an emphasis is placed on the numerical methods required to obtain robust behavior of the simulation. This formulation has been implemented into an open-source software package entitled PDSim written in the Python language. PDSim is the first open-source generalized compressor and expander simulation package and the complete source code is included in the Supplemental material. A piston expander is used as an example of the utilization of this framework. The framework has been applied to several positive displacement machines in the companion paper in order to demonstrate the flexibility of the approach (Ziviani et al., 2019).

Linear compressor technology is characterized by the absence of a crank mechanism, which has gained increasing attention in vapor compression systems due to its compactness and lower friction losses in comparison to conventional reciprocating compressors. Limited work was found in the open literature related to the development and in-depth validation of a comprehensive linear compressor dynamic model that couples thermodynamic aspects with both mechanical and electrical sub-models. This paper presents a comprehensive and generalized simulation model that is used to simulate the dynamic performance of a linear compressor. The model is based on mass and energy balance equations applied to open control volumes. The overall model is composed of several sub-models including the piston dynamics, electrical motor, valve dynamics, and leakage flows. The thermodynamic model and the sub-models are integrated into a compressor overall energy balance that describes the different heat flows and losses. The linear compressor model is able to predict both transient and steady-state behaviors of the piston movement, internal pressure and temperatures as well as the overall performance. Comparisons of predicted and measured mass flow rates as a function of operating frequency are also presented within this work.

Buildings are responsible for more than 73% of the total electricity usage in the United States and have a significant contribution to peak electrical demand of the power grid. Thus, proper buildin...

In recent years, variable electricity pricing has become available to residential consumers to incentivize demand reductions during midday peak hours. Thermal energy storage (TES) systems enable consumers to store cooling energy when demand is low and assist A/C operation during peak demand periods. However, the cost savings achievable using TES are highly dependent on how the system is operated for a given utility rate structure. This study investigates control strategies for a packaged chiller unit integrated with ice storage that leverage available residential utility rate structures in the U.S. to reduce consumer electricity cost. The present work describes the development and evaluation of a generalized rule-based control strategy inspired by the performance of an optimal controller that minimizes monthly electricity cost considering both time-of-use energy and demand charges. The generalized rule-based controller is compared against the optimal controller as well as to heuristic control strategies for TES that were originally developed for commercial buildings for a range of equipment cooling capacities, TES sizes, geographic locations, and residential utility rates. The total electricity cost is determined using a simulation model that includes models for the chiller unit, ice storage tank, and secondary loop components, along with a building load model. Results show that the generalized rule-based controller can approximate the performance of the optimal controller within 20% for all cases tested, and within 10% of the optimal cost in 53% of the cases tested. The controller also performs significantly better than the heuristic strategies for commercial buildings that were evaluated.

The energy savings potential of passive chilled beams in various climatic zones is analyzed in this study. A passive chilled beam model, developed based on full-scale experiments, is used as a system module in a whole building simulation tool to account for the convective and radiative effects of passive chilled beams. The model was validated with measurements from a field study in an open plan office equipped with multiple passive chilled beams. In addition, a parallel field study in an adjacent identical office space equipped with an air (VAV) system was conducted to compare the resulting energy consumption with the two systems. To further study the energy savings potential and indoor thermal conditions with passive chilled beams, four different chilled beam system configurations, combined with a parallel variable air volume system, were studied using the whole building model in different US climatic zones. The results showed that using a dedicated outdoor air system, or a separate chiller for the passive chilled beams, or a desiccant wheel after the cooling coil may result in significant relative energy savings depending on the location. The last option proved to be the most efficient, providing up to 12% for hot and humid climates and up to 20% for hot and dry climates –compared to a conventional variable air volume system. Finally, the radiative-to-total cooling effect of passive chilled beams varied between 7% and 8% and has only a small impact on energy savings and indoor thermal conditions.

The need for more energy efficient heating and cooling technologies will increase over the coming decades. Climatic concerns associated with greenhouse gas emissions as well as energy costs associated with powering heating and cooling systems necessitate more efficient systems. This work presents an alternative to traditional vapor compression cycles used for heating and cooling. The chemical looping heat pump system is an indirect chemical heat pump that utilizes the possibility of condensation after a chemical reaction to allow the working fluid's compression to take place in the liquid phase. A thermodynamic model of the cycle is developed using electrochemical cells as the reaction drivers of the cycle. Sensitivity analysis is used to identify the most critical components of the system. The chemical looping heat pump may provide a potential COP increase of over 20% when compared to traditional vapor compression systems for representative device efficiencies.

The power grid has seen record demand for frequency regulation capacity in recent years due to the increased employment of renewable energy resources worldwide. Building thermal loads are flexible and thus, can be used as regulation reserves with proper control strategies. Previous studies have shown that building HVAC equipment can provide high-quality power grid regulation service with PJM performance scores of up to 0.98 and buildings’ participation in the regulation market could bring significant economic benefits for building owners. However, the power flexibility in buildings is not persistent and the available HVAC regulation capacity has significant hour-by-hour variation due to building load and other operating constraints. This paper presents a regulation capacity reset strategy for HVAC regulation control that identifies the available regulation capacity and baseline power on the fly with real-time load and operation data. The strategy relies on a steady-state HVAC performance model derived from manufacturer performance data and implements a pseudo-optimization that seeks the maximum regulation capacity while respecting all operating constraints. The proposed strategy was implemented on a variable-speed rooftop unit (RTU) and validated with laboratory tests in psychrometric chambers. The test results show that the proposed reset strategy is effective in providing consistent high-quality regulation service with negligible impact on the indoor temperature control; the zone temperature deviation from the setpoint was within 0.3 °C for all the performed tests. The reset strategy was also simulated with a prototypical building diurnal load model to quantify the integrated regulation capacity for a typical summer day. Simulation results indicate the integrated HVAC regulation capacity throughout a summer day equals approximately 1/4 of the daily electrical energy use; and the estimated daily regulation credit can offset up to 26% of the daily HVAC electricity cost based on PJM historical prices.

Real-time power supply and demand balances are critical to ensure stable power frequency and quality power services. However, the growing integration of renewable energy increasingly challenges the power infrastructure because most of the renewable resources, e.g., solar and wind energy, are intermittent and difficult to predict. To meet stringent power frequency requirements, more fast reacting frequency regulation resources are being brought online among which grid-level batteries are the dominant ones. Buildings consume more than 73% of the electricity in the US, offering significant regulation reserve for the power grid. Variable-speed air-conditioning and heat pump systems are taking an increasing share of the market due to higher efficiency requirements imposed by federal agencies. In addition to efficiency benefits, variable-speed cooling/heating systems are also perfectly suited to provide frequency regulation service since these units can modulate their power continuously over a wide range. This paper presents a methodology and case study results for laboratory-based assessments of power frequency regulation service performance for variable-speed HVAC cooling equipment. The assessment methodology involves the use of both open- and closed-loop testing in psychrometric chambers. The open-loop testing quantifies the maximum possible performance of regulation services that is building independent, whereas the closed-loop testing provides representative performance for regulation services when the equipment must maintain comfort conditions for typical “virtual” buildings. Two air-conditioning units, a variable-speed packaged rooftop unit (RTU) and a split heat pump with an after-market variable-speed retrofit, were utilized for regulation tests performed in psychrometric chambers. Both open-loop and closed-loop regulation control strategies were developed and tested for this equipment. Open-loop test results showed that variable-speed HVAC equipment can provide high quality regulation service with performance scores of up to 0.98. With PJM historical wholesale energy and regulation prices, the estimated regulation credit can offset up to 48% of the electricity price. The closed-loop regulation control was shown to have negligible impact on indoor comfort with temperature fluctuations smaller than 0.1 °C. Power regulation also did not cause any efficiency degradation for the closed-loop tests. In fact, for more humid climate conditions, the electrical input for cooling can even be reduced with regulation control due to reduced moisture removal.

Carbon dioxide (CO 2 ) is commonly used in the ventilation control of buildings as a metric for indoor air quality (IAQ); however, few commercially available sensors exist that can reliably measure CO 2 while being low cost, exhibiting low power consumption, and being easily deployable for use in applications such as occupancy monitoring. This work presents an initial assessment of a chemiresistive CO 2 sensor that leverages a polymer blend of branched polyethylenimine (PEI) and poly(ethylene glycol) (PEG) as the CO 2 absorbing layer. Prototype sensors were assessed in a bench-top environmental test chamber at various ambient temperatures (22-26 °C), relative humidity levels (20-80%), and CO 2 concentrations (400-2,000 ppm), which are typical of indoor environments. The results show that with some further advancements, the proposed method could serve as an alternative to current commercially available CO 2 sensing technologies.

A load-based methodology for testing and rating the performance of unitary air conditioning equipment has been developed and demonstrated using a laboratory psychrometric chamber test facility. The...

Organic Rankine cycles (ORCs) are thermodynamic power cycles designed to generate work from a wide range of heat source conditions. In particular, low-grade waste heat recovery (WHR) ( ° C) can be effectively exploited with such systems. The efficiency of an ORC is highly dependent on its expander performance. In the low power output range ( ° C and 110 ° C, were investigated. The scroll expander achieved a maximum overall isentropic efficiency of 0.58 for the temperature source of 110 ° C, for the imposed specific volume ratio of 6.12 at rotational speed of 1600 rpm. For the same heat source, the maximum expander power output was 3.75 kW for an imposed specific volume ratio of 6.55 and rotational speed of 2500 rpm. Besides the experimental work, the performance of the expander was characterized by means of a semi-empirical model to break-down the different loss terms. A well known model available in the literature was extended to account for the major frictional losses in a scroll machine, i.e. bearings, tip-seals and other sources of friction. Additionally, an Artificial Neural Network (ANN) modeling approach was also proposed to achieve higher accuracy in mapping expander performance for use in system simulation. The experimental data and model source codes are provided as supplementary materials .

Single-screw type of machines has been widely used in the vapor compression industry for decades. More recently, such machines have been proposed as volumetric expanders in organic Rankine cycle (ORC) for low-grade waste heat recovery applications because of a number of advantages compared to other volumetric machines, e.g., twin-screw, scroll, rotary, reciprocating, such as balanced loading of the main screw, long working life, high volumetric efficiency, low leakages, low noise, low vibration, and a simplified configuration. In this paper, a thorough literature review is proposed to cover the available single-screw geometry models, which are mainly applied to single-screw compressors (SSC). Thus, the geometry model is extended to single-screw expanders (SSE). The validity of the analytic and numerical solutions for the calculation of the volume curves is discussed. The geometry model is applied to a 11 kW single-screw expander and both the profile generation and geometry calculations are validated.

Packaged air-conditioning systems, such as rooftop units (RTUs), are widely used in small commercial buildings in the United States. Although new regulations and standards require new air-conditioning equipment to have multi-speed supply fan operation, most of the installed units employ fixed-speed supply fans. The electrical energy usage associated with constant volumetric air delivery fan systems is on the order of half the total RTU electrical energy usage because supply fans operate continuously during the occupied periods in commercial buildings for ventilation. In addition, compressors within RTUs typically feature single- or two-stage capacity controls leading to frequent unit cycling and thus, deteriorated performance in energy efficiency. This paper presents relatively comprehensive assessment results for three RTU variable-speed retrofit options: 1) retrofits of fixed-speed supply fans with variable-speed fans for single-compressor-speed units, 2) replacement of existing fixed-speed RTUs with RTUs having two-speed compressors with variable-speed supply fans and 3) retrofits of RTUs with new RTUs that incorporate variable-speed compressors and fans (indoor and outdoor). The assessment study involves parametric simulations across different building types and locations in the U.S. to understand the performance variation with respect to location and building use. Performance improvements in terms of energy efficiency, operation cost and indoor comfort delivery for the various retrofit options are demonstrated compared to conventional single-stage RTUs with constant speed supply fans. The assessment results show that all three retrofit options offer significant fan energy savings, but much smaller savings potential for compressor and condenser fan electricity consumption. The variable-speed-unit retrofit provides the highest cost savings potentials, ranging from 40 to 250 $ per rated cooling ton per year depending on the building type and location. In addition to efficiency and economic benefits, the two-stage-unit and variable-speed-unit retrofits also lead to improved indoor humidity regulation performance due to reduced unit cycling.

Organic Rankine cycle with liquid-flooded expansion and internal regeneration (ORCLFE) has been demonstrated leading to improvements in system efficiency with respect to conventional ORC with internal regeneration in the heat input temperature range from 80 ∘C to 200 ∘C. From a previous study [1], it was concluded that the selection of the positive displacement expander, the working fluid and its interaction with the flooding medium, and the two separate heat inputs heavily impacted the exergetic conversion efficiency of the cycle, but they were not analyzed in depth. In this paper, a more detailed thermodynamic cycle model has been developed to include a physical-based expander model to evaluate the impact of built-in volume ratio, flooding ratio, mechanical losses and the solubility of the working fluid into the lubricant oil. As waste heat recovery from 100 ∘C to 150 ∘C is the main application of the ORCLFE under investigation, low-GWP refrigerants such as R1234ze(Z), R1233zd(E) and R1336mzz(Z) have been considered as potential replacements of R245fa. A single-screw expander, which has been previously characterized, has been employed to optimize the ORCLFE at three heat source temperatures (100 ∘C, 120 ∘C and 150 ∘C) for each refrigerant, in order to obtain more realistic performance predictions.

The performance of passive chilled beams is characterized in this study to understand their physical behavior when applied in real indoor environments. Two full-scale experimental studies were conducted to map the performance of passive chilled beams in two different experimental settings: (i) single passive chilled beam testing in a controlled laboratory environment and (ii) multiple passive chilled beams testing in a real open plan office setting. The experimental results were then used to develop regression models for predicting the total cooling capacity and chilled surface temperature of single and multiple passive chilled beams, as a function of water flow rate, water supply temperature, air temperature above the chilled beam and area-weighted uncooled surface temperature in the space. The developed models showed good agreement with experimental results and can be used in building energy modeling tools for system simulation using passive chilled beams. Finally, it was found that the conventional method of predicting the total cooling capacity of a passive chilled beam from individual beam laboratory tests or manufacturers’ catalogs may significantly underestimate the system performance in multi-unit configurations. These differences could influence optimal system sizing and commissioning and should be considered in future studies.

As much as 40% of the total load on air conditioning systems can be attributed to condensation dehumidification. However, new water vapor-selective membranes present a unique opportunity to greatly reduce the power requirements for moisture removal by avoiding phase change and have thus been ranked as a top alternative to traditional HVAC systems. To date, however, all such systems have relied on the assumption of constant temperature, even terming the technology “isothermal dehumidification.” This work proposes a membrane-based air cooling and dehumidification approach, referred to as the Active Membrane Energy Exchanger (AMX), which is the first to provide simultaneous, yet decoupled, air cooling and dehumidification. The suggested AMX configuration uses two vapor-selective membrane modules with a water vapor compressor in between them, using the second membrane module to reject vapor into the exhaust stream. Cooling and heating coils in each membrane module channel move heat between the air streams using a vapor compression cycle. A detailed steady-state, thermodynamic model is presented for the AMX integrated within a 100% outdoor air conditioning system. The AMX’s limiting parameters and design considerations like compressor efficiency are systematically analyzed for a broad range of outdoor air conditions and compared against standard and state-of-the-art dedicated outdoor air systems. This new high efficiency approach is found to outperform all other standard and state-of-the-art systems, achieving 1.2–4.7 times the COP over conventional dedicated outdoor air treatment. Lastly, a building simulation case study predicted cooling energy savings as high as 66% in hospital buildings with 100% outdoor systems in hot, humid climates.

The Hyperloop vehicle is a conceptual mode of transportation in which a pod travels at high speed through a low-pressure tunnel to minimize the aerodynamic drag. In the current paper, an aerodynami...

To achieve the U.S. Department of Energys brake thermal efficiency (BTE) goal for Heavy Duty Diesel Engine (HDDE) technologies, Waste Heat Recovery (WHR) by means of Organic Rankine Cycle (ORC) systems has been selected as a suitable solution. The current relatively high return on investment period of such technology needs to be improved by significant cost reductions to realize benefits on WHR for mobile applications. The performance of the ORC system under dynamic loads relies on the choice of the working fluid, the efficiency of its components (mainly expander) as well as the control strategy that optimizes the operation. A novel ORC architecture is proposed that uses the engine coolant as the working fluid. In particular, a fraction of the engine coolant, which is a mixture of water and ethylene glycol, is employed as working fluid through the ORC to recover waste heat from EGR (Exhaust Gas Recirculation) and part of the tail pipe exhaust gases. At the inlet of the expander, the mixture has mixed-phase conditions and a fixed volume ratio expander is employed to generate power output that can be fed directly to the engine crankshaft. Heat rejection is accomplished through the spare capacity of the engine radiator, which avoids the need for a separate condenser. To evaluate the feasibility of such system architecture, a thermodynamic steady-state cycle model has been developed to predict the potential increase of BTE under different engine loads as well as to understand the ORC performance. Parametric studies are carried out by varying the system pressure ratio, the internal volume ratio of the expander and the mixture quality at the expander inlet.

This paper deals with the operation and optimization of a down-scaled industrial organic Rankine cycle (ORC) for low-grade waste heat recovery. The system is a sub-critical regenerative ORC with a nominal power output of 11 kW. The ORC unit has been assembled using off-the-shelf components including three identical plate heat exchangers, a liquid receiver, a multi-stage centrifugal pump and a single-screw compressor adapted to operate as an expander. The experimental results are used to evaluate the influence of the expander performance on the behavior of the ORC system at nominal and part-load conditions. The matching between the volumetric expander and the system operating conditions is also analyzed. Results showed that in the case of SES36, both the expander efficiency and system performance were maximized for a pressure ratio between 7 and 9. In the case of R245fa, while the system efficiency achieved values similar to SES36, but the expander maximum isentropic efficiency was 17% lower. Two analyses are carried out to optimize the operation of the ORC unit with R245fa. At first, the insights gained by analyzing the experimental data are used to evaluate the theoretical matching between volumetric expander and the system maximum efficiency in terms of the Second Law of thermodynamics. Secondly, a control-oriented steady-state cycle model based on empirical correlations calibrated on the experimental results is developed. The model is used to implement a feed-forward control strategy based on predetermined steady-state points that allow to optimize the operation of the ORC unit. The frequency of the pump is used as controlled variable. The steady-state experimental data of both working fluids is provided in an electronic annex .

A figure of merit, V, to characterize the value of an AFDD maintenance tool is presented. The AFDD tool is fed data representing air-conditioners operating with and without faults across a range of conditions. The calculation of V considers the probability of each scenario occurring, the tool's response, and the resulting implications. These results are summed to give a typical average value for deploying the AFDD tool, as compared to maintenance being performed without AFDD. Case studies illustrate the calculation of V. Six AFDD tools and two idealized tools are evaluated. Five of the six real tools give negative V, meaning that their use causes more harm than good. The sixth shows approximately $10 benefit per nominal ton of air-conditioner capacity per typical maintenance visit. This represents about half of the maximum potential value. These results demonstrate the importance of measuring AFDD performance and the potential monetary benefits of AFDD.

Implementation of an ejector for expansion work recovery in transcritical Carbon Dioxide (CO2) cycles provides an opportunity to improve the efficiency of these environmentally-friendly refrigeration systems. However, literature outlining an approach to ejector design for a given application is lacking. This paper presents a tool to design a complete ejector applied in a vapor compression cycle. In this work, the developed design tool was validated using experimentally-derived polynomials at air conditioning conditions, then efficiencies were input to broaden the analysis to study a transcritical CO2 system with an ejector operating in the evaporating temperature range of −15 °C to 20 °C and gas cooler pressure in the range of 80–110 bar. The design tool allows for the calculation of the motive and suction nozzle throat diameters, the mixing section diameter, and the diffuser outlet diameter, as well as the lengths of each section, to output a full internal geometry of the ejector based on performance requirements. Individual component sub-models are presented within the proposed model structure. The model which forms the basis of the design tool was experimentally validated with a mean absolute error (MAE) between 3% and 4%. Additionally, the sensitivity of the ejector geometry and performance to component efficiencies, operating conditions, and component versus system optimization was investigated. The optimization and parametric studies conducted provided novel insights into the impact of desired efficiency and operating conditions on ejector geometry, thus allowing a designer to make decisions based on the tradeoff between ejector size and performance. For example, as the diffuser length increased by 5.1 mm to obtain an efficiency increase, to obtain a further efficiency increase of the same amount would require a 17.1 mm length increase in diffuser length. Potential model improvements and other future work are also discussed.

This article introduces a simple empirical model that can be readily implemented in building simulation programs to model the effects of refrigerant charge on air conditioner performance. Under- or over-charged refrigerant faults are relatively common and impact performance of equipment. It is important to understand their overall impacts in the context of whole building energy consumption and comfort in order to prioritize their importance relative to other building issues. This article develops an empirical model using some existing data. The model applies corrections to the rated values of cooling capacity, energy input ratio, and sensible heat ratio of existing air-conditioning equipment modeling approaches. Model predictions are compared with experimental results obtained from three air conditioners that have fixed orifices as expansion devices. The accuracy of the model is compared with a more complex empirical model and is shown to provide better accuracy for predicting cooling capacity and sensibl...

This paper focuses on the experimental and numerical characterizations of a single-screw expander for waste heat recovery organic Rankine cycle (ORC) applications. A down-scaled industrial ORC test-rig has been tested with two different working fluids, R245fa and SES36. The hot source inlet temperature has been set to 125 °C and the maximum expander inlet pressure was limited to 1200 kPa. A total of 102 steady-state points have been collected by varying the expander pressure ratio between 3 and 9 with rotational speeds in the range from 2000 rpm to 3300 rpm. The experimental results allowed to assess the relationship between internal built-in volume ratio and imposed expansion ratio at different rotational speeds with respect to shaft and overall isentropic efficiency as well as volumetric performance in terms of filling factor. Results showed that while R245fa allowed approximately a 10% higher power output, the single-screw expander was performing at higher isentropic efficiency with SES36 due to higher pressure ratio achievable under the given working conditions and system limitations which also led to a better matching between ORC system and volumetric expander performance. A semi-empirical model has been developed and calibrated to break down the expander internal losses in the case of R245fa. The model has been exercised to investigate the effect of potential design improvements on the overall performance. The friction losses played a major role in the total loss count followed by suction pressure drops and leakages. As a consequence, the effect of lubrication should be further investigated to reduce leakages and friction. This study demonstrates the potential of single-screw technology as volumetric expander for ORC applications.

Identification approaches applied to semi-physical thermal network structures, so called gray-box modeling approaches, are popular in building science for energy audits, retrofit analysis and advanced building controls, e.g. model predictive control. However conventional identification approaches applied to thermal networks fail when there are significant unmeasured heat gains that influence building responses. This paper presents a method to obtain improved gray-box building models from closed loop data having significant unmeasured disturbances. The method estimates both physical parameters of a building thermal network model and also a disturbance model that characterizes the unmeasured inputs. The performance of the algorithm is demonstrated using numerical and experimental results.

Penetration of advanced building control techniques into the market has been slow since buildings are unique and site-specific controller design is costly. In addition, for medium- to large-sized commercial buildings, HVAC system configurations can be very complex making centralized control infeasible. This paper presents a general multi-agent control methodology that can be applied to building energy system optimization in a “plug-and-play” manner. A multi-agent framework is developed to automate the controller design process and reduce the building-specific engineering efforts. To support distributed decision making, two alternative consensus-based distributed optimization algorithms are adapted and implemented within the framework. The overall multi-agent control approach was tested in simulation with two case studies: optimization of a chilled water cooling plant and optimal control of a direct-expansion (DX) air-conditioning system serving a multi-zone building. In both cases, the multi-agent controller was able to find near-optimal solutions and significant energy savings were achieved.

In the US, buildings represent around 40% of the primary energy consumption and 74% of the electrical energy consumption [U.S. Department of Energy (DOE). 2012. 2011 Buildings Energy Data Book. Energy Efficiency & Renewable Energy]. Incentives to promote the installation of on-site renewable energy sources have emerged in different states, including net metering programmes. The fast spread of such distributed power generation represents additional challenges for the management of the electricity grid and has led to increased interest in smart control of building loads and demand response programmes. This paper presents a general methodology for assessing opportunities associated with optimal load management in response to evolving utility incentives for residential buildings that employ renewable energy sources and energy storage. An optimal control problem is formulated for manipulating thermostatically controlled domestic loads and energy storage in response to the availability of renewable energy gener...

Automated fault detection and diagnosis (AFDD) tools are used to identify degradation faults that reduce the performance and life of air-conditioning equipment. A recent methodology has been developed to evaluate the performance of AFDD tools. The methodology involves feeding a library of input data to an AFDD protocol and categorizing the results. The current paper describes a study that has been conducted to assess the effect of using various input data sets in the evaluations. These input data sets include different distributions of fault type, fault intensity, and operating temperatures. Case study evaluations of three AFDD protocols in current widespread use are used to demonstrate the effects. The paper shows that evaluation results are sensitive to input data sets, and argues that data sets used in previously published studies should be improved to give higher fidelity evaluations. It concludes that for AFDD performance evaluation to be meaningful, the fault and operating conditions need to be controlled so that they connect to the anticipated deployment conditions. A related conclusion is that it is necessary to use simulation data, rather than laboratory measurement data, to conduct performance evaluation of AFDD.

This article presents a general approach for determining maximum monthly energy cost savings associated with optimal supervisory control for cooling in commercial buildings in the presence of utility rates that include both demand and time-of-use energy charges. The resulting tool has a month-long time horizon because of the nature of demand changes and is only useful for benchmarking the performance of simpler and shorter-term demand response and optimal control approaches. Attempts to solve this optimization problem using a centralized formulation failed and, therefore, the benchmarking problem was formulated as a dynamic optimization problem within a multi-agent control framework so that the monthly optimization problem is segmented into several sub-problems where each sub-problem involves system optimization for a shorter period of time, for example, a 1-day period. The daily-scale optimization involves determination of trajectories of zone set-point temperatures that minimize an integral cost functio...

Oil flooding is a technique that can be utilized in compression systems to achieve near-isothermal compression. This can lead to a boost in system efficiency and a reduction in compressor power consumption. In this paper a semi-empirical model for oil flooded compressors using liquid injection was developed. The model is validated with experimental data and integrated into a thermodynamic model of a vapor compression system with oil flooding and regeneration. The performance of the heat pump system is predicted and the semi-empirical model is used to identify and estimate the magnitude of the irreversibilities during the compression process. A method for generalizing the model for different working fluids is also presented. Using this model, design recommendations are made to improve the efficiency of the studied liquid flooded compressors for heat pump applications.

Metal hydrides have been investigated for use in environmentally friendly heat pumps. These systems operate using reversible adsorption and desorption of hydrogen from metallic compounds and can be incorporated in a cycle having a work input (compressor) or thermal energy input (generator). Some challenges faced by compressor driven metal hydride heat pumps are poor heat transfer in the metal hydride beds and high compressor discharge temperatures. To overcome these challenges, this article investigates the use of a metal-hydride slurry in conjunction with various isothermal compression techniques. Liquid-flooded, electrochemical, and liquid piston compressors were modeled and integrated into a system model in order to assess their impact on the performance of the slurry-based metal hydride heat pump system.

Empirical models of four chiller faults that can be applied within existing building models to study overall impacts are developed in this paper. The faults include overcharging, excess oil, non-condensables in refrigerant and water-side condenser fouling. A single generalized model structured was developed for these faults that forces predicted fault impacts to be zero with no fault and increase with increasing fault level. The models were trained and tested using available laboratory data for a water-cooled centrifugal chiller where all four faults were artificially introduced. The fault model behavior was studied and then they were integrated in hospital models from DOE commercial reference building models (Deru et al., 2011) and simulations were performed in different climates. The simulation results showed maximum increases of building electricity consumption, electricity peak demand and water consumption of the hospitals due to faults of 4.7%, 7.8% and 1.8% respectively. The fault impacts were found to be more severe in hotter and more humid climates.

Virtual sensors use data from low-cost measurements and calibrated models to provide outputs that would either be too expensive or impossible to measure directly. Virtual sensor technology has the potential to enable cost-effective implementation of advanced monitoring, diagnostic, and/or control features for buildings. While it is commonly known that the reliability of virtual sensors depends on the amount and conditions of calibration data, no methods have been presented that quantify the effect of the conditions of calibration data on virtual sensor output uncertainty. In this paper, a general method is presented for estimating the virtual sensor output uncertainty in terms of the uncertainty, conditions and amount of calibration data. The method is demonstrated with a power consumption virtual sensor for packaged air conditioning systems.

Refrigerant mass flow rate is an important measurement for monitoring equipment performance and enabling fault detection and diagnostics. This paper presents and evaluates three different virtual refrigerant mass flow (VRMF) sensors that use mathematical models to estimate flow rate using low cost measurements. The first model uses a compressor map that relates refrigerant flow rate to measurements of condensing and evaporating saturation temperature, and to compressor inlet temperature measurements. The second model uses an energy-balance method on the compressor that uses the compressor power consumption. The third model is developed using an empirical correlation for an electronic expansion valve (EEV) based on an orifice equation. The three VRMFs are shown to work well in estimating refrigerant mass flow rate for various systems under fault-free conditions with less than 5% RMS error. The combination of the three VRMFs can be utilized to detect and diagnose when the compressor and/or expansion device is not providing the expected flow.

The primary goal of this work was to develop, demonstrate, and evaluate a fault detection and diagnostics (FDD) system that incorporates integrated virtual sensors and fault impact models for decision support. FDD systems have the potential for improving energy efficiency along with reducing service costs. To achieve this goal, virtual sensors and fault impact models that only require low cost measurements are implemented to detect the cause of the fault, diagnose the severity of a fault and assess the performance reduction. The integrated virtual sensors can handle multiple simultaneous faults to isolate a specific fault from other faults. The fault impact models can decouple performance impacts of individual faults under multiple fault conditions. A complete diagnostic FDD system was implemented and demonstrated for a rooftop air conditioner. The test conditions included a wide range of fault and operating conditions with multiple simultaneous fault situations. Laboratory tests were performed to define reasonable thresholds for the FDD system with multiple simultaneous faults to assess whether RTU service should be performed. The FDD system provided the following diagnostic outputs for and impacts of: 1) loss of compressor performance, 2) low or high refrigerant charge, 3) fouled condenser, 4) evaporator filter, and 5) faulty expansion device.

“Free cooling” with cool outside air can meet or reduce a building’s cooling loads in an energy efficient manner when conditions are favorable. For rooftop air conditioning units (RTUs), this is achieved with air-side economizers that are integrated with the packaged RTU. However, current RTU economizers are independently controlled at the unit level and may not fully leverage the free resource for economic benefit especially when multiple units serve a common zone of a building, such as in retail stores. That is, the independent control of multiple RTUs could lead to some units operating with mechanical cooling even when the outdoor air temperature or enthalpy is low enough to meet the load with economizer cooling. This paper presents a model-predictive control (MPC) approach to optimally supervise multiple RTUs in order to fully utilize the free cooling resource. The algorithm is applied to the evaluation of the energy savings potential for a case study building served by four identical RTUs under various climate zones over a cooling season.

Thermal energy storage based (TES-based) reverse cycle defrosting method is a feasible way to reduce energy requirements for defrosting of cascade air source heat pumps (CASHPs). The energy stored in the phase change material based heat exchanger (PCM-HE) is the heat source for both the high stage cycle (HSC) and low stage cycle (LSC) during the TES-based reverse cycle defrosting process, thus the heat provided to the HSC and LSC are coupled, affecting the defrosting performance in LSC and heating performance in HSC. To shorten the defrosting time and maintain the indoor thermal environment, the heat distribution between HSC and LSC during TES-based reverse cycle defrosting was explored. It was found that the heat distribution pattern was mainly influenced by the refrigerant flowrate. Therefore, controlling the refrigerant flowrate of HSC and LSC during the TES-based reverse cycle defrosting is a possible way to regulate the heat distribution between HSC and LSC. In this paper, an experimental investigation of the control strategy for TES-based reverse cycle defrosting of CASHPs is presented. The defrosting performance and indoor heating performance during TES-based defrost operation with LSC compressor frequencies of 60 Hz, 75 Hz, 90 Hz, and 105 Hz were tested. Then HSC EEV openings of 80%, 90% and 100% at the optimized LSC compressor frequency of 90 Hz were considered. It was concluded that the optimal TES-based reverse cycle defrosting strategy is to maintain the LSC compressor frequency at 90 Hz and the HSC EEV opening at 80% for the equipment considered in this study. The defrosting time was 440 s and the average indoor heating capacity was 5.23 kW. The heat provided to the HSC and LSC accounted for 38.7% and 61.3% of the total heat released by PCM-HE, respectively.

When organic Rankine cycles (ORC) are employed to convert waste heat into work, the thermal efficiency is not recommended as a key performance metric because waste heat recovery and power output are not generally maximized at the point of peak efficiency. In such an application, maximization of the net power output should be the objective. Two alternative cycle configurations that can increase the net power output from a heat source with a given temperature and flow rate are analyzed and compared to a baseline ORC. These cycle configurations are: an ORC with two-phase flash expansion (TFC) and an ORC with a zeotropic working fluid mixture (ZRC). A design-stage ORC model allowed a consistent comparison of multiple ORC configurations with finite capacity of the source and heat sink fluids. Simulation results indicated that the TFC offered the most improvement over the baseline ORC, but required a highly efficient two-phase expansion. The ZRC shows improvement over the baseline as long as the condenser fan power requirement is not negligible. At the highest estimated condenser fan power, the TFC is no longer beneficial. Finally, a partial-evaporating ZRC (PE-ZRC) has also been considered to reduce the volume ratio requirements of a flash expansion.

Growth of a frost layer on an evaporator surface due to low evaporator temperature as well as moisture contained in surrounding air deteriorates performance of a refrigeration system significantly and requires significant energy for defrost. Many studies have been performed to model the heat and mass transfer phenomena in an attempt to have insight and accurate prediction. However, many models form nonlinear algebraic differential equations which require iterative numerical solvers. Computationally efficient but accurate models are needed in order to evaluate overall system performance. The objective of this paper is to introduce a modeling approach to overcome the problem. A solution strategy based on an enthalpy-based reformulation and linearization method will be presented. Comparisons of the proposed and detailed model results for both flat plate and finned tube heat exchangers are provided. The proposed modeling approach is around 10 times faster than reference models while maintaining comparable accuracy.