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Flow maldistribution in evaporators can lead to significant degradation of capacity and efficiency of vapor compression equipment. A significant amount of work has previously been done to mend these issues. For variable air flow maldistribution, refrigerant compensation was proposed to reduce the performance degradation. For fixed air side maldistribution, refrigerant circuitry modifications were proposed to significantly reduce the effects of the maldistribution. However, no work has been found on modifying the refrigerant circuitry to make it less vulnerable to varying air side maldistribution. The purpose of this paper is to fill this gap in open literature. The performance of the new, interleaved circuitry approach and an active refrigerant flow control is compared to the standard circuitry for different cases of maldistribution. The results show that the interleaved circuitry recovers less of the performance losses than equalization of the exit superheats. However, the implementation cost in an actual system is expected to be significantly lower and the long term reliability is expected to be much better than for an active control approach.

Previous research indicates that a scroll compressor with multiple vapor injection ports has the potential to significantly improve the energy efficiency and maintain high heating capacity of vapor compression heat pumps, particularly at low outdoor temperatures. Based on a tradeoff between cost and benefit, a dual-port vapor injected compressor prototype was chosen for testing to provide the experimental results. The overall performance of the heat pump system employing the compressor prototype is analyzed. Coupling the compressor testing results into a three-stage expansion vapor injection flash tank cycle model, the operating injection pressure can be determined under a certain working condition. The results showed that both lower and higher injection pressures and the corresponding injected mass flow rates decreased with an increase of pressure ratio of discharge pressure to suction pressure. Comparing the COP of the vapor injection cycle with the one of a baseline cycle, the improvement in COP increased with an increase of the pressure ratio and goes up to 19% at a pressure ratio of 8 and a condensing temperature of 43.3°C. In addition, the proposed technology leads to higher energy efficiency and less degradation in heating capacity at low ambient temperatures. For Minneapolis, the seasonal efficiency (heating) of the vapor injection cycle was 13% higher than that of the standard vapor compression cycle.

A methodology for evaluating the performance of fault detection and diagnostics (FDD) tools for unitary air-conditioners has been developed (Yuill and Braun 2013). The methodology uses laboratory measurements of systems with and without faults to test FDD tools’ effectiveness. A gray box modeling method capable of modeling systems with faults was developed by Cheung and Braun (2013a and 2013b) to provide input data, as an alternative to using laboratory data that had been collected. The simulation method was validated by direct comparison with experimental data, but a comparison of FDD evaluation results provides a more direct and useful validation of the model for its intended purpose. Eight different systems have been modeled using Cheung and Braun’s method. Six FDD tools were evaluated using both experimental and modeled inputs under the same environmental and fault conditions. The fault conditions include non-standard charging, heat exchanger fouling, loss of compressor volumetric efficiency, liquid line restriction, and the presence of non-condensable gas in the refrigerant. The model’s performance is characterized by comparing its outputs from the evaluation – false alarm rates, misdiagnosis rates, missed detection rates, and rates of undiagnosed faults – with the results based upon experimental data. The model is found to be highly suitable for its purpose.

This paper presents an application of tower fan control for optimization of the performance of chiller plants combined with free cooling on the condenser water side. Mathematical models including all the main components of an existing cooling plant were developed and implemented in MATLAB. Simulation results include a mapping of the performance of the plant working in free cooling mode which was used to select control parameters for free cooling operation. Then a mapping of the plant operating with chillers was developed to find the correlation between load and near-optimal air flow, which is the basis of the near-optimal tower control (NOTC) strategy. Finally, simulations were carried out using three consecutive years of historical data to predict the performance of the plant under three different control strategies: 1) tower fan control aiming to keep the temperature of the water supplied to chiller condensers at a constant set point (current control strategy), 2) NOTC and 3) NOTC and free cooling combined. Comparison of the performance of the plant with the baseline (constant condenser water temperature) shows that significant savings can be achieved through the implementation of NOTC along with free cooling. It is expected that the methodology and results of this study provide a useful framework for optimization of cooling plants.

Model based predictive control (MPC) in building HVAC systems incorporate predictions of weather and occupancy to determine the optimal operating setpoints. However, application of MPC strategies to large buildings might not be real time feasible due to the large number of degrees of freedom in the underlying optimization problem. Decomposing the problem into several smaller sub-problems to be solved in parallel is one way to circumvent the high computational requirements. Such an approach, termed Distributed MPC, requires certain approximations about the underlying sub-problems to converge to a consistent solution thus leading to a trade off between computational load and optimality. In this paper, we present a simulation based evaluation for a Distributed MPC formulation for a case study based on a medium sized commercial building. Results indicate that distributed MPC can offer near optimal control at a fraction of the computational time that centralized optimization based MPC requires while maintaining occupant comfort. Comparison with a few other viable control algorithms will be performed and merits and drawbacks of each approach pointed out.

Air conditioning contributes significantly to building energy consumption in hot climate regions. In addition to greater cooling requirements in hot climates, cooling equipment efficiency decreases with increasing outdoor temperature. Therefore, it is advantageous to develop improved technologies that can achieve higher efficiency at high ambient conditions. In this paper, two novel compression technologies are investigated for application in high ambient temperature air conditioning via simulations. These technologies are liquid flooded compression with regeneration and vapor injected compression with economizing. The systems are modeled using the EES software and compared with a baseline conventional vapor-compression cycle that utilizes R410A as the refrigerant. The cycle enhancements are considered for a number of refrigerant alternatives, including R410A, propane (R290), R32, and R1234yf. Parametric studies are conducted for air conditioning design conditions to predict the improvements in coefficient of performance (COP) for both system configurations with the various refrigerants. The simulation results show that the two novel technologies provide improvements in air conditioner performance and lower compressor discharge temperatures at high ambient temperatures. With respect to compressor discharge temperature, the vapor injection technology is superior to the oil flooding concept for the investigated working fluids. The COP comparisons indicate that oil flooding only improves the system performance when using the refrigerant R1234yf with a 14% increase in COP, whereas the vapor injection leads to significant improvements for all refrigerants with a maximum improvement of 21.5% for the refrigerant R410A.

Heating capacity and coefficient of performance (COP) of conventional air-source heat pumps decreases towards lower ambient temperatures. In addition, high discharge temperature cutout at the compressor outlet might limit the operation of the heat pump at very low ambient temperatures. Oil injected into the compression chamber absorbs part of the heat generated during the compression process, which can result in significant reduction of the compressor discharge temperature. Discharge temperature therefore decreases with increasing injected oil mass fraction, especially at low ambient temperatures. Therefore, oil injection allows the application of air-source heat pumps in regions with very low ambient temperature in winter. Additionally, oil injection decreases the compressor power consumption by providing better sealing and lower friction during the compression process. Furthermore, if oil injection is combined with a regenerative heat exchanger, the system performance of a vapor compression system can be improved significantly. The work presented on this paper shows the experimental results of a 5-ton (17.6 kW) R410A packaged heat pump which was retrofitted with an oil injected compressor and regenerator. The effect of different oil mass fractions on the system performance was investigated under standard AHRI 210/240 heating test conditions. According to the results, more than 7% system COP improvement was observed compared to the baseline system.

The use of liquid-flooding in the compression and expansion of non-condensable gas in scroll compressors and expanders enables the possibility of quasi-isothermal working processes. Liquid-flooded scroll machines were installed in a fully-instrumented Liquid-Flooded Ericsson Cycle test rig to conduct entire cycle performance tests. In addition, detailed compressor and expander performance data was obtained. Oil mass fractions of up to 92% and 76% were added to the gas entering the scroll compressor and expander respectively. The overall isentropic efficiency of the scroll compressor based on the shaft power with flooding was up to 73% and the volumetric efficiency was above 92%. For the expander, the best overall isentropic and volumetric efficiencies achieved were 66% and 105% respectively. The mechanistic model presented in the companion paper was validated against the experimental data for both the compressor and the scroll expander with good agreement, though the agreement is better for the scroll compressor.

In two companion papers, simulation models for the working processes of liquid-flooded scroll compressors and expanders have been developed and validated against experimental data. In this study, analytic models are presented for the modes of irreversibility generation in the liquid-flooded scroll compressor including built-in volume ratio maladjustment, pressure drop and leakage. A thermodynamic model is used to derive the ideal volume ratio for a liquid-flooded compressor, which is higher than that of dry compression. An optimum set of built-in volume ratio and scroll base circle radius is found which maximizes the overall isentropic efficiency by minimizing the leakage and pressure drop irreversibilities. The irreversibility generation models are used to optimize a scroll compressor for the Liquid-Flooded Ericsson cycle application. The model predicts scroll compressor overall isentropic efficiency of over 80% (based on the shaft power) at an oil mass fraction of 88%.

A detailed mechanistic model has been developed for scroll compressors and scroll expanders with liquid flooding for application to the Liquid Flooded Ericsson Cycle. This model is based on the integration of a set of differential equations that arise from the conservation laws. The impact of the flooding liquid on the working process is also included through mixture properties as well as treatment of two-phase flow pressure drop. With the working process known, it is then possible to solve for the compressor power, total mass flow rate and other model outputs. The fine level of detail in the model allows for the investigation of the impact of design changes to the scroll machines. This model can also be readily extended to allow for the analysis of liquid flooded vapor compression refrigeration.

The scroll wraps of a scroll compressor play a very important role in the compressor’s efficiency due to their impact on leakage and leakage irreversibilities. In this work, a short summary of the scroll compressor geometry is presented. Using the geometry of the scroll wraps, an optimal scroll wrap geometry is derived for a given set of constraints on compressor displacement, scroll wrap thickness and volume ratio. This optimal scroll wrap geometry can be used as a starting point for further optimization in a detailed compressor model that includes the effects of mechanical losses.

While models have previously been developed to investigate scroll compressor performance with a single injection port, the model described in this paper explores the effect of multiple injection ports on the performance of a novel rotary spool compressor. The model includes the effects of heat transfer and leakage and is numerically solved to predict the compressor power consumption and mass flow rate. The injection ports are modeled assuming that saturated vapor is injected at a specified pressure and the timing of the injection process can be controlled. Running at a speed of 1907 rpm with R-22 as the working fluid, an evaporating pressure of 391 kPa, an inlet temperature of 7.6°C, and a discharge pressure of 1890 kPa, the model predicts that adding a single injection port will provide a 12% increase in the coefficient of performance (COP) of the cycle. Adding a second injection port increases the COP by an additional 4% compared to the cycle with a single port, or 16% over the baseline performance of the cycle without economization. The compressor model is also used to investigate the effect of injection pressure, injection port location, and injection port diameter on economized cycle performance.

This paper utilizes the component models described in a companion paper to develop a full system inverse model for normal and faulty performance of vapor compression cooling and heating equipment. Algorithms to simulate different faults, such as loss of refrigerant charge, compressor valve leakage, liquid line restriction, etc. are presented. Component model parameters were estimated from laboratory experimental data without complete knowledge about the component characteristics. The system model was tuned to offset the bias that resulted from model simplifications. The method was carried out with data from a 3-ton R410a packaged unit with fixed orifice expansion, operating with faults such as incorrect refrigerant charge and heat exchanger fouling. The system model outputs had good agreement with the experimental data. The impacts of faults on performance determined through simulation are also presented.

As the market for ductless heat pump systems (DHPs) has grown so has the need to represent their performance in building simulation programs to enable the study of their effect on building energy consumption and to assist in the building design. For computationally efficient building assessments, quick computation for power consumption under different external effects is necessary. In this paper, data from various heating and cooling tests on a 9500 Btu/h (2782 W) unit are presented. In both operation modes, the minimum and maximum capacity under different environmental conditions, the indoor unit power consumption and the part-load performance were modeled from the data. A defrost model was also constructed to adjust the heating part-load model for defrost operation and a sensible heat ratio model was built to capture the humidity removal performance of the DHP in cooling. Good agreement between the measurement and prediction was found.

In this work, a residential air conditioning compressor designed for vapor injection has been modified in order to inject large quantities of oil into the working chamber in order to approach an isothermal compression process. For a range of different oil injection scenarios, the result is a monotonic decrease in compressor discharge temperature as the oil injection rate is increased. In addition, the pumped refrigerant mass flow rate tends to increase as the oil injection rate is increased.

An empirical frictional correction factor to the isentropic nozzle model has been developed for application to leakage modeling in scroll, rotary and other similar compressors. This correction factor is derived by calculating the leakage mass flow with a compressible, variable area, real gas properties model and then correlating the ratio of this flow to the isentropic nozzle model prediction. The ratio of flows is correlated to the Reynolds number, a characteristic length and the leakage gap width. A representative selection of fluids and geometries are employed, and the correlations predict at least 93% of the detailed model results within an absolute error band of 20%.

The proposed hybrid control employs a primary expansion valve that provides most of the pressure drop, while small secondary balancing valves in the distributor lines to the evaporator circuits adjust the refrigerant flow to provide equal circuit exit superheats. This paper presents experimental results for application of hybrid control of expansion valves to a 5-ton R410A domestic heat pump. Baseline performance data was taken with an electronic expansion valve (EXV) to determine the best possible performance without using individual circuit flow control. After that, secondary balancing valves were inserted into the distributor lines to complete the hybrid control scheme. The same tests as done with the EXV were repeated to determine the achievable performance improvement. Ice-up tests at high outdoor humidity were conducted to determine the influence of the control scheme on frost build up and system performance. For repeatable results, additional tests with a partially blocked evaporator coil were conducted. It was found that during operation with the EXV, even with a clean coil, substantial maldistribution occurred leading to individual circuits with significantly different refrigerant exit conditions. With the EXV control scheme maintaining an overall exit superheat, some circuits had two-phase refrigerant at the exit while other circuits had a high degree of superheat. As a result, the evaporation temperature was lower than would occur for more even distribution leading to a performance penalty. In addition, the maldistribution resulted in uneven ice build-up during the ice-up test. The tests with the hybrid control scheme indicated that the maldistribution can be avoided and better overall performance achieved. Similar tests as for the domestic heat pump have been conducted for a 3-ton large room cooling system; refer to companion paper Bach et al.(2012).

Fault detection and diagnostics (FDD) tools are being increasingly applied in air-conditioning systems. There are many different protocols used in these FDD tools, so an important question to ask is: how well do the protocols work? This paper describes the ongoing development of the first standardized method of evaluation for FDD protocols applied to air-cooled vapor compression air-conditioning systems. The general approach is to feed a library of data – including temperatures, pressures, and humidity – to a candidate FDD protocol, and observe the responses of the protocol. The library includes data from units operating with no fault or with single faults. The faults include undercharge or overcharge of refrigerant; impaired heat transfer in the condenser or evaporator; liquid line flow restriction; presence of non-condensable gas in the refrigerant; and compressor valve leakage. The outputs from the test are categorized, and statistics and figures are generated. A preliminary evaluation of RCA, a public domain FDD protocol, has been conducted using laboratory measurement data from 13 air conditioning systems. The rates of (a) no response, (b) correct, (c) false alarm, (d) missed detection, and (e) misdiagnosis show that the protocol performed poorly. This underscores the need for standardized FDD evaluation methods.

An organic Rankine cycle (ORC) is a power cycle employing an organic working fluid. The term ORC is also applied generally to any Rankine cycle with a low-grade heat source (80° – 300°C). Because ORC are often employed in small-scale applications, use of positive displacement equipment is favored over the centrifugal units used in large-scale power plants. A key feature of a positive displacement expander is its built-in volume ratio. An ORC with a scroll-type expander is studied experimentally. Data for steady state tests of the ORC are presented according to a proposed steady state standard. It is shown that the adiabatic efficiency of the expander can be fully characterized by its filling factor and the expansion volume ratio imposed across it. In particular, the peak adiabatic efficiency occurs near a filling factor of unity and an expansion volume ratio near the built-in volume ratio of the expander. The influence of the expander’s performance on cycle efficiency is considered. A procedure is presented which allows prediction of cycle performance based on knowledge of the expander efficiency versus expansion volume ratio, cycle operating conditions, and working fluid. Using this procedure, an optimal expander can be chosen for a set of cycle operating conditions based on its peak efficiency and built-in volume ratio.

The use of variable-speed compressors in heat pumps and air conditioners has increased in recent years in order to improve comfort and energy efficiency. At the same time, there is a trend towards embedding more sensors in this type of equipment to facilitate real-time energy monitoring and diagnostics. Although compressor mass flow rate and power consumption are useful indices for performance monitoring and diagnostics, they are expensive to measure. The virtual variable-speed compressor sensor (VVC) was developed to estimate mass flow rate and power consumption using inexpensive temperature sensors and embedded models. The mass flow and power consumption at maximum compressor speed and rated superheat are first correlated with suction and discharge pressure. These pressures are estimated using virtual pressure sensors that use condensation and evaporation temperatures measured on return bends within the heat exchangers. The mass flow and power predictions are corrected for different speeds and inlet superheat heats using additional correlations. The VVC can predict mass flow rate and power consumption within RMS errors of ± 5% and ± 3%, respectively, under normal (no fault) conditions. The VVC also works well under conditions where faults in the system exist, since the VVC isolates these two estimates from other possible external faults such as condenser or evaporator fouling. One of the inputs to the model is compressor frequency, which can be difficult to measure for technicians in the field. However, for an embedded application, the compressor frequency would generally be available from the motor controller. Alternatively, the VVC could be used to estimate compressor frequency if mass flow rate or power consumption was provided as an input from another independent virtual sensor.

In residential heat pump systems, the motivation for secondary loop systems is to allow for the use of flammable or toxic refrigerants with lower global warming potentials than the currently employed HFC refrigerants. The addition of radiant panels as integral building components (embedded in concrete at construction or attached to the underside of wood flooring) is becoming more common. Combining the large surface area of the radiant panel and an efficient primary loop, a hydronic secondary loop heat pump system can greatly outperform a conventional air-to-air heat pump. The improvement in coefficient of performance is as much as 38% over a conventional air-to-air heat pump when the secondary loop hydronic system is employed. Due to the large area of the radiant panel, the condensing temperature of the primary loop for the hydronic secondary loop heat pump can be reduced by as much as 5°C at high ambient temperatures.

This paper presents an efficient and robust parameter training methodology, based on a previous approach for inverse building modeling that utilizes a simplified state-space approach. One new element of this training methodology is that some seasonal effects, such as variation of window transmittance at different times of the year, are taken into consideration and captured during the training process. In addition, a mixed-mode training approach is developed that allows the use of a combination of data obtained when cooling or heating is occurring with the zone temperature under control at setpoint and when the zone temperature is floating during periods of no load. To obtain a “nearly” global optimal model, a multi-start search method was found to be robust and provide good computational efficiency and accurate results. The training methodology is implemented to model three zones of Building 101 at the Navy Ship Yard in Philadelphia, Pennsylvania.

Model based predictive control (MPC) is increasingly being seen as an attractive approach in controlling building HVAC systems. One advantage of the MPC approach is the ability to integrate weather forecast, occupancy information and utility price variations in determining the optimal HVAC operation. However, application to largescale building HVAC systems is limited by the large number of controllable variables to be optimized at every time instance. This paper explores techniques to reduce the computational complexity arising in applying MPC to the control of large-scale buildings. We formulate the task of optimal control as a distributed optimization problem within the MPC framework. A distributed optimization approach alleviates computational costs by simultaneously solving reduced dimensional optimization problems at the subsystem level and integrating the resulting solutions to obtain a global control law. Additional computational efficiency can be achieved by utilizing the occupancy and utility price profiles to restrict the control laws to a piecewise constant function. Alternatively, under certain assumptions, the optimal control laws can be found analytically using a dynamic programming based approach without resorting to numerical optimization routines leading to massive computational savings. Initial results of simulations on case studies are presented to compare the proposed algorithms.

The hybrid control, as proposed by Kim et al. (2008), employs a primary expansion valve that provides most of the pressure drop, while small secondary balancing valves in the distributor lines to the circuits of the evaporator adjust the refrigerant flow to provide equal circuit exit superheats. This paper shows the experimental results for the application of the hybrid control of expansion valves for a 3-ton R404A large room cooling system. Data with the inbuilt expansion device, a pressure compensated TXV, was taken for a limited number of tests. Baseline data with an electronic expansion valve was taken to determine the best possible performance without using individual circuit flow control. After that, secondary balancing valves were inserted into the distributor lines to complete the hybrid control scheme. The same tests as done with the EXV were repeated to determine the achievable performance improvement. Ice-up tests at high indoor room humidity were conducted with all control schemes to determine the influence of the control scheme on frost build up and system performance. For repeatable results, additional tests with partially blocked evaporator coil were conducted with the hybrid and EXV control scheme. It was found that for TXV and EXV, even with a clean coil, substantial maldistribution occurs. This maldistribution lead to different usage of the individual circuits in terms of area fraction used for evaporation of, and area fraction used for superheating of refrigerant. With the EXV control scheme, the evaporation temperature had to be decreased to obtain sufficient superheat on the circuits that did not feed liquid into the suction header in order to evaporate the liquid of the circuits that fed liquid into the suction header. In addition, this resulted in uneven ice build-up and very poor controllability, which was especially noticeable as severe hunting when using the TXV in the ice-up test. With the hybrid control scheme, the surface usage for evaporation on all circuits was larger than 75% for most of the time, which resulted in a higher evaporation temperature and by that in a greater COP and larger capacity.

In conventional vapor compression systems, electronic expansion valves (EXVs) are used for refrigerant flow control. Subcooled refrigerant enters the expansion device and is expanded to the evaporation pressure while the valve opening is modified to achieve the desired mass flowrate. The relationship between the inlet and outlet conditions, the opening, and the mass flowrate has been extensively studied, e.g. by Park et al. (2007) and appropriate empirical correlations have been developed. However, for certain operating conditions (e.g. low refrigerant charge) or applications that generally have two-phase inlet conditions (e.g. balancing valves used in a hybrid control scheme as proposed by Kim et al. (2008)), these correlations are not applicable, since even low inlet vapor fractions lead to a significant reduction of the valve mass flowrate at a given opening. This paper proposes a continuous correlation that can be used for both two-phase and subcooled valve inlet conditions. The benefit of the continuity is that there is a smooth transition between subcooled and two-phase inlet conditions, which is essential for control and simulation purposes. The new correlation employs the Buckingham-Pi theorem as proposed by Buckingham (1914). The selected dimensionless Pi-groups describe opening of the valve, subcooling, inlet and outlet pressures, driving pressure difference across the valve, inlet density, surface tension, and viscosity. The data that was used to determine the coefficients of the correlation was taken on a dedicated valve test stand, which was sized for the per-circuit capacity of a typical 5-ton R410A heat pump and a 3-ton R404A large room cooling system. The purpose of these tests was mainly to map the valves for the low pressure drops, high inlet qualities and large valve openings that occur when they are used as balancing valves in a hybrid control approach. Two commercially available valves of different rated capacity were tested. Due to the much higher valve capacity for subcooled inlet conditions, valve openings of less than 5% occurred in that case. This led to an accuracy of the correlation for these points that is less than what typically can be found for correlations with subcooled inlet conditions in the open literature. However, for two-phase flow inlet conditions, the resulting RMS of 1.0 g/s for the 8-PI correlation is sufficiently small to use the approach for estimating the refrigerant mass flow and using the EXV as a virtual flow sensor. The limitations of this approach in practical applications, as well as possible applications in fault detection and diagnostics are shown for application as balancing valves within a 5-ton R410A heat pump and a 3-ton R404A large room cooling system.

Outdoor air economizers (OAE) control the amount of air drawn into a building in order to reduce energy usage and improve indoor air quality. Air-side economizers are required by energy standards such as ASHRAE Standard 90.1 in many commercial applications because of their energy savings’ potential. However, many economizers do not operate as designed because of damper, sensor, and other faults which can actually lead to greater energy use. Fault detection and diagnostics (FDD) tools have been developed to identify faults when they occur, however, no general methodology for assessing these tools has yet to be proposed. This paper presents and applies a general OAE FDD assessment method. FDD tools described by Seem and House (2009), Schein et al. (2003), and Brambley et al. (1998) are analyzed with respect to various OAE faults. Additionally, these FDD methods are assessed with the method using simulated and experimental data with both single and multiple simultaneous faults.

This paper presents generic equipment models that capture the performance of families of similar heat pumps equipment and that can be used in building simulation programs. Mapping has been carried out for families of single and dual compressor speed ducted split systems and the correlations are being implemented as components for building simulation. The units mapped ranged in nominal capacity from 1 to 5 tons (3.5 - 17.6 kW) and 2 tons to 5 tons (7.0-17.6 kW) for the single and dual speed units, respectively. Equipment models for building simulation were generated based on the ASHRAE secondary toolkit direct expansion model. Single sets of correction factor equations were found for each family in heating mode and cooling mode by first aggregating normalized performance data of all the units in each family and then mapping the normalized performance characteristics using the ASHRAE toolkit model (Brandemuehl, 1993). The dual speed unit model was split into two parts; one set of coefficients for low speed and another for high compressor speed. These models are based on modern equipment currently on the market and provide a useful update to the currently available standard direct expansion air conditioner models in building simulation programs. The generalized models generated in this study were compared to established performance models based on the ASHRAE toolkit model for cooling and the DOE 2.1 RESYS routine for heating mode operation.

Refrigerant mass flow rate is an important measurement for monitoring equipment performance and enabling fault detection and diagnostics. However, a traditional mass flow meter is expensive to purchase and install. A virtual refrigerant mass flow sensor (VRMF) uses a mathematical model to estimate flow rate using low-cost measurements and can potentially be implemented at low cost. This study evaluates three VRMFs for estimating refrigerant mass flow rate. The first model uses a compressor map that relates refrigerant flow rate to measurements of inlet and outlet pressure, and inlet temperature measurements. The second model uses an energy-balance method on the compressor that uses a compressor map for power consumption, which is relatively independent of compressor faults that influence mass flow rate. 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. Each of the three mass flow rate estimates can be utilized to diagnose and track the following faults: 1) loss of compressor performance, 2) fouled condenser or evaporator filter, 3) faulty expansion device, respectively. For example, a compressor refrigerant flow map model only provides an accurate estimation when the compressor operates normally. When a compressor suction or discharge valve is leaking and the compressor is not delivering the expected flow, the energy-balance or EEV model can provide accurate flow estimates. In this case, the flow differences provide an indication of loss of compressor performance and can be used for fault detection and diagnostics, as will be demonstrated in this paper.

An organic Rankine cycle (ORC) is often used in waste heat recovery applications. These are typically small-scale applications where cycle thermal efficiency is low, and the benefits of traditional cycle enhancements (such as reheat stages or feed-water heaters) do not typically outweigh the costs required to implement them. An ORC with flooded expansion and internal heat regeneration is an alternative enhancement that provides comparable benefits at reduced cost and complexity. The improvement in efficiency for the ORC with flooded expansion and internal regeneration is analyzed for several working fluids and for two flooding media: water and Zerol 60 compressor lubricant. It is shown that internal regeneration alone provides most of the efficiency enhancement for dry working fluids (R600a, n-Pentane, and R245fa). n-Pentane is shown to offer the most efficient cycle even without flooded expansion in most cases. A quantitative comparison is given between the proposed cycle and the reheat and feedwater heater cycles with internal regeneration. In applications where a hydrocarbon may not be appropriate as a working fluid, R245fa and R717 show promise as alternatives. R717, which shows the most benefit from flooded expansion and internal regeneration, requires this enhancement in order to be competitive with the dry working fluids.

The paper presents performance characteristics and an approach for mapping the performance of a variable-speed ductless heat pump system (DHP) operating in heating mode. This type of equipment is gaining popularity and is not currently included within building simulation programs that are typically used for design and LEED (Leadership in Energy and Environmental Design) certification. Steady-state performance maps are needed for these simulation tools. Experiments were conducted on a DHP at various conditions and regression analysis was implemented to generate a model. Comparisons were made with data which were not used to derive the model and good accuracy was demonstrated. The resulting performance map was used to demonstrate trends in performance with operating conditions.