Variable-speed Heat Pump Research Articles

A variable speed water-to-water heat pump model used for ground-source applications

The main objective of this study is to model variable speed water-to-water heat pumps ( VSHP ) and to examine the impact of the operation of such devices on ground heat exchanger sizing and energy consumption when they are used in ground-source applications. In the first part of the paper, a complete physics-based steady-state model of a variable-speed water-to-water heat pump is briefly presented. A performance map approach is also used by modifying an existing TRNSYS variable speed heat pump model to provide a minimum speed of operation and a better representation at part load. Simulation results over a heating season on a residential ground source VSHP indicate that the energy coverage (i.e., percentage of annual heat supplied by the heat pump) increases at a faster rate than the effect coverage (i.e., percentage of peak building heat supplied by the heat pump) for a small value of the effect coverage. For example, for effect coverage of 60%, the energy coverage is ∼ 93%. It is also shown that the normalized length of the ground heat exchanger varies linearly up to effect coverage of 60% where it is equal to 75% of the value encountered for effect coverage of 100%.

A novel methodology to assist manufacturers in the heat exchanger sizing for variable-speed heat pumps based on part load conditions and economic assessment

Matts Bäckstöm assessed the selection criteria of heat exchangers for single-speed heat pumps in 1940, and his methodology and conclusions have been kept practically unchanged for almost a century. However, heat pump systems have evolved, especially with the introduction of variable-speed heat pumps. These new systems introduced a new degree of freedom to the heat pump design – the compressor speed – which is not considered by traditional design criteria.This study proposes novel design criteria to optimize the heat exchanger size when the heat pump unit can work under part-load conditions. The proposed method models how heat exchangers and variable-speed compressors perform under different loads and sizes. It considers part-load requirements based on climatic data (which are typically available in the standards) and considers economic factors such as initial investment and operation costs (which are dependent on the components's size). Given the mentioned information, a multivariable optimization algorithm is implemented to find the optimum heat exchanger size that minimizes the total cost. To validate the proposed approach's effectiveness, it was exemplified by determining the optimum size of a coil and a plate heat exchanger for a domestic variable-speed heat pump. The results demonstrate that using traditional criteria could lead to over-dimensioning heat exchangers in variable-speed heat pumps which could incur in an increase of costs of 5% for the studied case.

Building Energy Flexibility Assessment in Mediterranean Climatic Conditions: The Case of a Greek Office Building

The EU energy and climate policy has set quantitative goals for decarbonization based on the energy efficiency and the evolution of energy systems. The utilization of demand side flexibility can help towards this direction and achieve the target of higher levels of penetration in regard to intermittent renewable energy production and carbon emission reduction. This paper presents a simulation-based assessment of thermal flexibility in a typical office building in Greece, which is a representative Mediterranean country. The use of variable speed heat pumps coupled with hydronic terminal units was evaluated. The research focused mainly on the evaluation of energy flexibility offered by energy stored in the form of thermal energy by utilizing the building’s thermal mass. The demand response potential under hourly CO2eq intensity and energy prices was investigated. The flexibility potential was evaluated under different demand response strategies, and the effect of demand response on energy consumption, operational costs, CO2eq emissions and thermal comfort was analyzed and discussed. The results showed that both control strategies based on both the CO2eqintensity signal and spot price signal have, in some cases, the potential for cost and emission savings, and in other cases, the potential to depreciate in terms of emissions and cost the increase of energy consumption due to load shifting.

Integrated Thermodynamic and Control Modeling of an Air-to-Water Heat Pump for Estimating Energy-Saving Potential and Flexibility in the Building Sector

Reversible heat pumps are increasingly adopted for meeting the demand for space heating and cooling in buildings. These technologies will play a key role not only in the decarbonization of space air conditioning but also in the development of 100% renewable energy systems. However, to assess the achievable benefits through the adoption of these technologies in novel applications, reliable models are needed, capable of simulating both their steady-state operation and dynamic response at different conditions in terms of heating loads, outdoor temperatures, and so on. The operation of heat pumps is often investigated by highly simplified models, using performance data drawn from catalogs and paying scarce attention to the critical influence of controllers. In this respect, this paper proposed an integrated thermodynamic and control modeling for a reversible air-to-water heat pump. The study considered a heat pump alternatively equipped with variable-speed compressors and constant-speed compressors with sequential control. The developed modeling was then used to investigate the operation of an air-to-water heat pump serving an office building in Italy. Results show that the model provided insights into the transient operation of variable-speed heat pumps (e.g., the settling time). Regarding constant-speed heat pumps, the model provided hints of interest to the control engineer to prevent, in the examined case study, the risk of quick compressors cycling on low-load heating days or when low-temperature heating devices are supplied. Finally, using a control strategy based on a heating curve for the variable-speed heat pump, results show the potential for a sensible increase in the average coefficient of performance, from 17% up to 50%.

Fault detection model for a variable speed heat pump

The impact of the COVID pandemic has resulted in many people cultivating a remote working culture and increasing building energy use. A reduction in the energy use of heating, ventilation, and air-conditioning (HVAC) systems is necessary for decreasing the energy use in buildings. The refrigerant charge of a heat pump greatly affects its energy use. However, refrigerant leakage causes a significant increase in the energy use of HVAC systems. The development of refrigerant charge fault detection models is, therefore, important to prevent unwarranted energy consumption and {CO}_{2} emissions in heat pumps. This paper examines refrigerant charge faults and their effect on a variable speed heat pump and the most accurate method between a multiple linear regression and multilayer perceptron model to use in detecting the refrigerant charge fault using the discharge temperature of the compressor, outdoor entering water temperature and compressor speed as inputs, and refrigerant charge as the output. The COP of the heat pump decreased when it was not operating at the optimum refrigerant charge, while an increase in compressor speed compensated for the degradation in the capacity during refrigerant leakage. Furthermore, the multilayer perception was found to have a higher prediction accuracy of the refrigerant charge fault with a mean square error of ± 3.7%, while the multiple linear regression model had a mean square error of ± 4.5%. The study also found that the multilayer perception model requires 7 neurons in the hidden layer to make viable predictions on any subsequent test sets fed into it under similar experimental conditions and parameters of the heat pump used in this study.

Impact of lubricant in the performance of variable speed heat pumps working with R290

Modulating the speed of the compressors and adapting its capacity to the required load has led to a decrease in the annual energy consumption in many applications. However, in the compressor, having adequate lubrication at low speeds usually implies discharging too much lubricant at higher speeds. Although necessary for compressor operation, lubricating oil acts as a contaminant in the rest of the system. Consequently, manufacturers have to consider the oil circulation rate (OCR), as it limits the speed range of the compressor. In a previous study, a compressor was tested over its speed range (30 – 110 Hz), working with R290 and POE68 as a lubricant. Experimental data confirm the increase of OCR with speed and suggests that working with R290-POE68 could imply higher OCR values compared to other refrigerant-oil mixtures. In this study, the impact of the OCR in the performance of a R290 heat pump was theoretically assessed. Three contributions were studied: The heat transfer coefficient (HTC) reduction in the evaporator, the effect of refrigerant being solved in oil and finally, and the impact of oil in the compressor. It was found that OCR values of 5% could decrease the coefficient of performance (COP) by more than 20%.

Development of an accumulator liquid-level estimator to enable zero-superheat control and active charge management in vapor-compression systems

Vapor-compression system efficiency increases with reduced superheat but excessively low superheat settings may cause wet compression and jeopardize reliable system operations. Accumulators are commonly used to intercept liquid refrigerant before it reaches the compressor and can also be used as a refrigerant reservoir to assist active charge management. For zero-superheat and active charge control, accurate measurement or estimation of the accumulator liquid level is required. This paper presents an accumulator liquid level estimation approach based on a nonlinear state observer combined with a gray-box dynamic model of the evaporator and accumulator. This approach affords virtual detection of the liquid level in real-time from refrigerant pressure and temperature measurements, which are readily available in modern vapor-compression systems, and supports zero-superheat and dynamic charge control to maximize system efficiency. Both numerical and experimental tests of the proposed liquid level estimation and charge control strategies were carried out with a 3-ton variable-speed heat pump. The tests have demonstrated the accuracy of the accumulator liquid level estimator and shown that the zero-superheat and active charge control strategy could improve the system efficiency by up to 10.5%.

Optimal day-to-day scheduling of multiple energy assets in residential buildings equipped with variable-speed heat pumps

The largest parts of energy demand within the residential sector are used for space heating (SH), domestic hot water (DHW) and space cooling (SC). The thermal and electrical needs can be mainly covered by energy systems that consist of Photovoltaics (PVs), solar thermal collectors (STCs), heat pumps (HPs) and flexibility sources, e.g. battery energy storage (BES) and thermal energy storage (TES). In this paper, various control strategies for the optimal day-to-day operation of residential energy systems that consist of a variable-speed HP, STCs, PVs, as well as BES and TES units are examined. The proposed strategies are based on the concepts of self-consumption maximization (SCM) and time-of-use arbitrage (ToUA) considering priorities on BES and TES charging. A typical Greek single-family building is assumed examining multiple criteria for the assessment of the rule-based approaches. The results demonstrate that priority on TES charging can lead to higher system performance than on BES charging. Regardless of the number of ToU tariff zones, during off/mid-peak periods the upper energy needs of the SH/SC trajectory should be fulfilled only in case of excess PV power, otherwise the cost of imported energy increases significantly. Both SCM and ToUA strategies can lead to rates of self-consumption and self-sufficiency that deviate in average per about 4% from the optimal values. Moreover, the ToUA-based strategies can approach the minimum cost of imported energy with a relative increase of 22% compared to the optimum. However, the minimum thermal discomfort due to space temperature cannot be achieved without the prediction of the daily SH/SC demand. Furthermore, the proposed strategies can reach the minimum thermal discomfort due to DHW losses, while leading to lower BES degradation rates than the optimization. Finally, from the multi-objective optimization for the first year of system operation, it is verified that the optimal dimensioning of PV, BES and TES is considerably affected by the type of rule-based strategy selected.

Common fault effects on a natural refrigerant, variable-speed heat pump

The European heat pump market is growing every year as European legislation moves towards the use of natural refrigerants that have less global warming potential than conventional refrigerants. Even though heat pumps are very efficient, the appearance of hard-to-detect faults increases energy consumption. The present study emulated some of the most common faults on a variable-speed heat pump charged with propane. The faults tested were evaporator fouling, compressor valve leakage, liquid line restriction and refrigerant overcharge. The effect of these faults on performance and several heat pump features are described. Evaporator fouling was the fault with least COP degradation and compressor valve leakage with the most (7% and 56% COP reduction, respectively). The results are compared with the results from a comparable study. The heat pump covered the demand generated by nearly all the faults at the expense of increasing the power consumption. The features of each fault are summarized in a chart, which could be used as table for diagnosing faults.

A hierarchical gray-box dynamic modeling methodology for direct-expansion cooling systems to support control stability analysis

In this paper, a gray-box dynamic modeling approach for direct-expansion cooling systems is presented. The overall approach incorporates a multi-stage training procedure that consists of 1) identification of component sub-models from quasi-steady-state performance data, 2) system model integration with estimation of refrigerant charge and 3) fine tuning of thermal capacitances of the evaporator and condenser to capture the system dynamic responses. Compared to traditional physics-based models, the proposed modeling approach has advantages including reduced engineering efforts in the model development phase, improved computational efficiency and enhanced prediction accuracy. The modeling method was validated using a 3-ton variable-speed heat pump and proved to be capable of accurately predicting the system transient behaviors over a wide range of operating conditions. The established dynamic model was then applied for control stability analysis, with a specific goal of determining a proper control execution time step. The case study results showed that the stable control execution time step could change significantly, from 3 sec to 19 sec, as the operating conditions and control settings vary, and a proper selection of the execution time step is critical to ensure stable and reliable operations.

Assessing the performance limits of a variable-speed residential heat pump system

Most of the ducted split-type heat pump systems in the world feature fixed-speed compressors and fans. To meet forthcoming minimum energy rating requirements, reduce operational costs of the heat pump, and increase environmental sustainability, the efficiency of heat pump systems must be improved. Variable-speed equipment offers significant advantages for load modulation and has the ability to increase the seasonal performance significantly. Additionally, novel electrical motor technologies, such as permanent magnet (PM) motors can reduce the power consumption of the drives by up to 25-55% compared to the widely used permanent split capacitor (PSC) motor.In this study, a ducted fixed-speed heat pump system with a cooling capacity of 17.6 kW and a seasonal coefficient of performance (SCOP) rating of 4.11 is analyzed to quantify the theoretical performance limits that could be obtained by increasing the motor efficiency of the condenser fan, the evaporator fan and the compressor. Furthermore, the performance increase by replacing the fixed-speed components with variable-speed equipment is evaluated. A detailed charge-sensitive system simulation model that was previously developed by the authors and validated using experimental data obtained with the same unit was employed to carry out parametric studies to evaluate the impact of variable-speed compressor and indoor/outdoor fans on the SCOP rating.The study showed a 0.9% improvement of SCOP with every percentage increase of compressor motor efficiency. The dependence of the evaporator fan and condenser fan motor efficiency on SCOP was lower, but considering the generally low motor efficiencies of these components, a noticeable efficiency gain can be achieved. In variable-speed equipment, the motor efficiency of the component at minimum operational point was the significant factor on SCOP. The motor efficiency at intermediate and maximum speed had only marginal effects on the SCOP. This showed the importance of equipment with high motor efficiencies at low speeds for ducted split-system heat pumps. The change of the fixed-speed system to a fully variable-speed heat pump resulted in a high gain of SCOP, proving the superiority of a variable-speed system. Overall, the model predicted an SCOP gain of 35% when changing the system to a fully variable system with state-of-the-art high efficient motors.

A robust gray-box modeling methodology for variable-speed direct-expansion systems with limited training data

• A robust training method against limited training data is proposed for DX units. • The methodology identifies the most influential parameters to estimate. • More estimation parameters result in degraded parameter accuracy. • Model prediction accuracy is highly dependent on training data size. Compared to physics-based models, the gray-box modeling approach has clear advantages including reduced engineering efforts in the model development phase, higher computational efficiency, improved accuracy and reduced uncertainties. In this paper, we present a gray-box steady-state modeling methodology for variable-speed direct-expansion systems that is robust against limited training data. The methodology incorporates a dual-stage training scheme, where component models are identified separately, and then integrated through continuity constraints to establish a system model trained on a higher hierarch. To further improve model reliability and robustness, the training methodology incorporates two commissioning techniques, i.e., significance ranking and de-correlation, to down-select the estimation parameters according to the quality of the training data. To demonstrate the efficacy, the proposed modeling methodology was applied to a 3-ton variable-speed heat pump. Qualities of the identified models were evaluated in terms of parameter accuracy and model prediction accuracy against variable training data sizes. Robustness of the methodology was demonstrated in achieving reasonable model accuracies with limited training data.

Centralized and Decentralized Optimal Control of Variable Speed Heat Pumps

Utility service providers are often challenged with the synchronization of thermostatically controlled loads. Load synchronization, as a result of naturally occurring and demand-response events, has the potential to damage power distribution equipment. Because thermostatically controlled loads constitute most of the power consumed by the grid at any given time, the proper control of such devices can lead to significant energy savings and improved grid stability. The contribution of this paper is the development of an optimal control algorithm for commonly used variable speed heat pumps. By means of selective peer-to-peer communication, our control architecture allows for the regulation of home temperatures while simultaneously minimizing aggregate power consumption, and aggregate load volatility. An optimal centralized controller is also explored and compared against its decentralized counterpart.

Improving the energy flexibility of single-family homes through adjustments to envelope and heat pump parameters

With the increase of fluctuating renewable resources both on the grid and locally on buildings, a need arises for buildings to be flexible such that their energy demand can be modified to match energy generation. This study aims to characterize key approaches to increase the flexibility of a single-family home subject to a number of climate conditions. The home is modeled in TRNSYS in 5 climates, and building thermal mass, heat pump characteristics, and water heating setpoints are adjusted to assess the impact of each on the building's flexibility. The key metric used to assess flexibility is the cumulative absolute residual load over a year, where the residual load is defined as the difference between the average hourly demand of electricity and the average hourly available electricity. A control scheme is employed to implement demand response, but overall energy demand and thermal comfort are also considered throughout the year to assess effectiveness of the demand response approach. Results presented on 21 variations to the base house show the benefit of thermal mass in maintaining thermal comfort while changing thermostat setpoints and demonstrate the benefit of variable speed heat pumps for meeting available electricity supply. In the best case, a 36% decrease in residual load over the year is found for a simulation in Gaithersburg, MD compared to the base case. Results from this work suggest best approaches to take when developing new housing stock if consideration is needed for how well the buildings can respond to fluctuations in energy generation from intermittent sources such as wind and solar.

Smoothing control of solar photovoltaic generation using building thermal loads

Distributed energy resources such as solar photovoltaic (PV) systems are seeing significant expansion in power grids worldwide. However, serious system integration issues have arisen including voltage fluctuations and accelerated aging of voltage regulation devices, when adopting mass amounts of volatile energy resources onto legacy distribution networks originally designed for unidirectional power flow. Building heating, ventilation and air-conditioning (HVAC) systems, collocated with the distributed generation, can be considered as flexible loads due to the inherent building thermal inertia. HVAC systems can be proactively controlled to improve voltage regulation of distribution networks with high PV penetrations. This paper presents a smoothing solution that modulates HVAC power in response to volatile PV generation as a means to mitigate fluctuations in the net demand and generation. To demonstrate the effectiveness and evaluate performance gains, hardware-in-the-loop (HIL) tests were carried out using a 3-ton variable-speed heat pump. The HIL tests leveraged a building thermal dynamic model and a steady-state power flow model for a 33-bus distribution network to capture realistic indoor thermal responses and distribution voltage variations during PV smoothing control. Test results showed that the developed strategy was effective in reducing variations of net demand and generation with negligible impact on indoor comfort. More than 55% reductions of voltage fluctuation were achieved and tap operations of voltage regulators could be fully or partially eliminated with proactive PV smoothing.

Dehumidification performance of a variable speed heat pump and a single speed heat pump with and without dehumidification capabilities in a warm and humid climate

Conventional air conditioning systems in houses respond to thermal loads by means of controlling dry-bulb temperature through the thermostat. As part of the process to control temperature, dehumidification is also provided. However, as houses are becoming more efficient, supplemental dehumidification is often necessary for homes located in hot and humid climates to control relative humidity intentionally. This study compared the dehumidification performance of a residential air conditioning system working in three operations modes to emulate three different systems: a system with a variable speed mode, a single-speed system with an enhanced dehumidification mode, and a single-speed system operating in a traditional or normal cooling mode. With operation mode changes achieved through software, this study constituted a novelty in the topic of humidity control by using a single machine, with the same exact physical set-up to directly compare the dehumidification performance of three types of systems.Two types of days were of interest in the study, hot and humid days (summer season) and mild and humid days (Fall shoulder season). After assessment of the dehumidification performance, the variable speed mode was able to maintain relative humidity between 50% to 52% on summer days. In the single-speed with enhanced dehumidification, a slightly less effective humidity control was achieved on summer days with the mode keeping the relative humidity between 53% to 55%. In the normal cooling mode, which resembles a conventional system, the humidity levels were controlled between 55% to 60%. In the shoulder season, the variable speed and enhanced dehumidification modes maintained the relative humidity between 55% to 58% and 53% to 56% respectively. In the shoulder season, the normal cooling mode kept the indoor relative humidity near or above 60%. In terms of dehumidification efficiency expressed as a function of the amount of water condensate per unit of energy, the variable speed was determined to be more efficient than the other modes.

Virtual Inertia Control of Variable Speed Heat Pumps for the Provision of Frequency Support

The growth in the integration of converter interfaced renewable energy has reduced the system inertia, which threatens system stability due to high rate of change of frequency (RoCoF) and frequency nadir issues unless steps are taken to mitigate it. There is a need to provide sufficient fast frequency response to maintain adequate inertia in the system. This paper investigates the capabilities of a variable speed heat pump to provide an emulated inertial response. This paper presents a virtual synchronous machine control for a variable speed heat pump that provides support for grid frequency regulation over the inertial response time frame. A small-signal model with the transfer function of the variable speed heat pump is developed to analyse the effectiveness and feasibility of providing virtual inertia at the device and grid level, respectively. Furthermore, the small-signal model is validated using hardware in the loop simulation. Finally, the aggregated frequency response and virtual inertia contribution by a population of the heat pumps are evaluated and quantified in an urban distribution system.

Mixed-integer model predictive control of variable-speed heat pumps

Heating and cooling buildings cause a large and growing portion of the world’s carbon emissions. These emissions could be reduced by replacing inefficient or fossil-fueled equipment with efficient electric heat pumps. Recent research has shown that variable-speed heat pumps (VSHPs) could also provide a variety of power system services, such as price- or carbon-based load shifting, peak demand reduction, emergency demand response, and frequency regulation. By providing some or all of these services, VSHPs could facilitate the integration of wind and solar power into the grid, accelerating decarbonization of other economic sectors. However, the provision of power system services poses new and challenging VSHP control problems. Model predictive control (MPC) is a promising approach to these problems. MPC combines weather and load predictions, online optimization, and a mathematical model of a VSHP’s performance under varying ambient conditions. With few exceptions, existing VSHP models neglect two important features that arise in practice: (1) dependence of efficiency and capacity on compressor speed and indoor and outdoor temperatures, and (2) inability of VSHPs to operate at low compressor speeds. In this paper, we develop a new VSHP control model that considers both features. The model expresses the VSHP’s power consumption and heat production as affine functions of the driving temperatures and compressor speed. The model also includes a binary variable that constrains the VSHP to be either turned off, or operating within a range of acceptable compressor speeds using mixed-integer programming (MIP). This VSHP model and optimization strategy is combined with neural network based heat load prediction in MPC simulations. These simulations suggest that this approach could reduce energy costs by 9–22% and carbon emissions by up to 22%, relative to MPC with existing VSHP models.

Experimental Testing of Variable Speed Heat Pump Control Strategies for Enhancing Energy Flexibility in Buildings

Thermal mass of buildings and domestic hot water tanks represent interesting sources of thermal energy storage readily available in the existing building stock. To exploit them to their full potential, advanced control strategies and a coupling to the power grid with heat pump systems represent the most promising combination. In this paper, model predictive control (MPC) strategies are developed and tested in a semi-virtual environment laboratory setup: a real heat pump is operated from within a controlled climate chamber and coupled with loads of a virtual building, i.e., a detailed dynamic building simulation tool. Different MPC strategies are tested in this laboratory setup, with the goals to minimize either the delivered thermal energy to the building, the operational costs of the heat pump, or the CO2 emissions related to the heat pump use. The results highlight the ability of the MPC controller to perform load-shifting by charging the thermal energy storages at favorable times, and the satisfactory performance of the control strategies is analyzed in terms of different indicators, such as costs, comfort, carbon footprint, and energy flexibility. The practical challenges encountered during the implementation with a real heat pump are also discussed and provide additional valuable insights.

Optimal Price Based Demand Response of HVAC Systems in Multizone Office Buildings Considering Thermal Preferences of Individual Occupants Buildings

Thermal energy capacity of buildings can be coupled to power networks via heating, ventilating, and air-conditioning (HVAC) units. Optimizing the operation of HVAC systems in multizone buildings is a challenging task, as occupants have different thermal preferences dependent on time-varying indoor and outdoor environments. To estimate the social cost of demand response (DR), building aggregators need to assess occupants’ thermal discomfort (TD), which is related to their productivity outcomes. This paper proposes a price-based DR strategy for multizone office buildings to co-optimize the energy cost of HVAC units and the TD levels of occupants. To overcome simplified TD representations, a mobile interface is developed that grants occupants the ability to indicate their personal thermal preferences. These preferences are modeled using artificial neural networks, which are explicitly and directly integrated into the optimal DR scheduling. In addition, we evaluate the thermal response of a multizone office building to the operation of a variable speed heat pump (VSHP). Using the models of occupants’ TD and building thermal response, the optimization problem for the proposed DR strategy is formulated and solved with mixed-integer linear programming. The case study results verify that the proposed strategy successfully optimizes VSHP operations and occupants’ TD levels, mitigating the risk of occupant interruption to the optimal DR schedule.