Rooftop Units Research Articles

Development and validation of an intelligent load control algorithm

The renewable generation technologies form a significant (>20%) fraction of grid capacity, however their generation capabilities remain variable in nature. Therefore, utilities will be forced to maintain a significant standby capacity to mitigate the imbalance between supply and demand. Because more than 75% of electricity consumption occurs in buildings, building loads can be used to mitigate some of the imbalance. This paper describes the development and validation of an intelligent load control (ILC) algorithm that can be used to manage loads in a building or group of buildings using both quantitative and qualitative criteria. ILC uses an analytic hierarchy process to prioritize the loads for curtailment. The ILC process was developed and tested in a simulation environment to control a group of rooftop units (RTUs) to manage a building’s peak demand while still keeping zone temperatures within acceptable deviations. The ILC algorithm can be implemented at a low cost on a supervisory controller without the need for additional sensing. By anticipating future demand, the process can be extended to add advanced control features such as precooling and preheating to alleviate comfort when operation of the RTUs is curtailed to manage the peak demand.

Optimal control of indoor-air cooling in buildings using a reduced order model

In this work, we use Computational Fluid Dynamics (CFD) to generate the distributed dynamic responses of temperature and moisture in a restaurant that correspond to perturbations of input variables. The CFD model is validated with experimental data of the roof top units return temperatures. A Reduced Order Model (ROM) is developed by approximating the responses to these perturbations using a linear time-invariant model. The resulting indoor-air model is coupled to a dynamic envelope model with longer time scales. Assuming the outside temperature distribution and the occupants' loads are known throughout the day, optimal control is applied to our coupled model to minimize the cooling power subject to local and global comfort constraints. The results show that the applied method is more efficient and comfortable than the results given by the experimentally measured response of a conventional set-point control strategy.

Soft-repair technique for solving inherent oversizing effect on multiple rooftop units in commercial buildings

Rooftop packaged air-conditioning units (RTUs) have been intensively utilized in commercial buildings for providing space heating and cooling. They serve over 60% of the commercial building floor space in the U.S. Specifically, oversizing is an inherent issue practically caused by over design of mechanical engineers. With field test studies, oversizing can be up to 100% leading to high energy penalty. Although there are locally advanced control technologies utilized to improve the overall efficiency performance of RTUs, they are invasive approaches to interrupt normal operations and require experienced service teams for preventative maintenance causing high cost installation and service costs. The article proposes a novel and non-invasive methodology for permanently reducing the oversizing caused by non-optimal design or faulty design called “soft-repair”. The technique composes of coordination control and oversizing analysis to ultimately eliminate the fault impact without retrofitting original control. Control algorithms are systematically developed to mainly reduce oversizing effect utilizing simplified instantaneous building load for approximately quantify actual building load. With the control algorithm tested by a building simulation platform, the oversizing effect can be decreased at least 30% by comparing oversizing parameter signature of the original oversizing system with the improved results of soft-repair implementation because a suitable number and time operations of RTUs are automatically computed based on the actual building load and the soft-repair algorithm. With the decrease of the oversizing effect and energy penalty, the improve results lead to energy savings and extended life cycle of compressors, condenser fans and supply fans.

Transactional network: Improving efficiency and enabling grid services for buildings

Because of a need to reduce electricity consumption and to make electricity generation cleaner, there is an impetus to improve the energy efficiency of the building stock and widespread installation of distributed variable renewable energy generation. Soon, a significant fraction of transportation energy will be in the form of electricity. Significant gains in building operational efficiency are possible because between 20% and 30% of the energy consumed in commercial buildings is typically wasted because of poor or inefficient building operations. To address these problems, the U.S. Department of Energy's Building Technologies Office is supporting the development of the concept of a “transactional network” that supports energy, operational, and financial transactions between building systems (e.g., rooftop units), and the electric power grid using applications, or “agents,” that reside either on the equipment, on local building controllers, or in the Cloud. This article describes the transactional network concept, the platform, and two example agents/applications within VOLTTRON™ that improve operational efficiency and enable grid services for packaged air conditioners and heat pumps (rooftop units). It also describes the results from testing the concept at some demonstration sites.

Household Characteristics and Housing Deficits of Low-Income Renter Households in Housing Poverty: Focused on the 2014 Korea Housing Survey

This study explored the characteristics, housing deficits and expectations for support programs of low-income renter households in housing poverty in order to provide housing policy development and improvement information. Microdata of the 2014 Korea Housing Survey was utilized as secondary data for this study. A total of 2,508,672 low-income renter households (weighted count) in the bottom 40% income ranks of entire general households living in private rental units were selected as study subjects. The major findings were as follows. (1) One point four percent of the subjects were living in non-house living quarters (e.g., shanties, vinyl greenhouses, container houses, and mud huts), 1.4% were in dosshouses, 9.9% were in basements, semi-basements or rooftop units, and 8.2% were in sub-standard units. (2) Among the households whose housing costs burdens were measurable, 75.7% were found to have housing cost burdens to pay 30% or more of their household income towards housing costs (rental costs and maintenance costs), but only 7.5% of the burdened households received a housing voucher. (3) Eighty-one percent were found to be in a housing poverty status as defined by the researcher; in addition, low-income renter households in housing poverty in Seoul tended to have a greater proportion of households headed by females, the elderly, and be persons with low-education or disabilities. (4) Households in housing poverty showed greater expectations for financial support and/or extended provisions of public rental housing than other low-income renter households.

Development of Coordination Control for Multiple Rooftop Units

Rooftop Units (RTUs) have been widely applied in providing space heating and cooling for commercial buildings. In total, they serve over 60% of the commercial building floor space in the U.S. Even through the current control approaches of a RTU can maintain the zone temperature corresponding to a set point temperature, it performs inefficiently due to several factors such as limited sensing capability, non-coordinated local control, inherent oversizing effects and so on. In addition to unnecessary power consumptions, the current control and operation technology on RTUs also lead to the space humidity problem, equipment efficiency degradation, and premature failure. To solve aforementioned problems and enhance the overall system performance, this paper presents the development of a coordination control technique for improving the system operations of multiple RTUs used in light commercial buildings with an open space. In the control algorithm, simplified building models were developed to potentially estimate the instantaneous building load. Utilizing this model-based technique, sequence control strategy is designed to automatically select suitable mode operations of a RTU including economizing, heating, cooling and ventilation mode while synchronizing with the supply fan control and damper operations. Using a developed building simulation platform implemented on Matlab software, the developed coordination control is applied in reducing energy penalty caused by an inherent oversizing problem on multiple RTUs. With the findings, the control algorithm can be further used as a soft-repair for temporally fixing faulty operations and improper commissioning of multiple RTUs such as excessive or insufficient air flow, outdoor air leakage, stuck dampers and simultaneous heating and cooling (RTU fighting).

Virtual in-situ calibration method in building systems

Abstract Sensors play an important role in guiding building systems to achieve desired operation and efficiency. However, sensors are subject to continuous degradation and failures over time. Although a periodical calibration is needed, it is exceptionally difficult and/or impractical to many sensors with a conventional manual approach. Uncalibrated problematic sensors could significantly compromise the systems' performance and lead to unintended loss of energy efficiency in buildings. We propose a methodology, termed virtual in-situ calibration, to solve this critical issue. It is developed by mathematically extracting the characteristics of essential aspects involved in a calibration, including the environment assessment, benchmark establishment, and uncertainty quantification. A case study of a supply air temperature sensor in rooftop units illustrates the implementation process; the erratic uncertainty is reduced from ± 19.2 °C to ± 0.7 °C after the virtual in-situ calibration. The calibration method can be implemented online to significantly improve the reliability of sensor networks in buildings.

Dynamic programming based approaches to optimal rooftop unit coordination

While buildings served by multiple rooftop units are widely prevalent in retail and food industries, optimal coordination among rooftop units has received little attention. Currently, most buildings utilize plain feedback or heuristic strategies to determine rooftop unit operations. This article presents a switched dynamic programming based approach for optimal scheduling of rooftop unit operations. Using a mid-sized restaurant served by multiple rooftop units as an example, a switched affine dynamic system model is first presented. The rooftop unit coordination control problem is then formulated as a switched optimal control problem with nontrivial switching cost. A dynamic programming based method is proposed for the solution of the switched optimal control problem, which has reasonable computational complexity and can be implemented either as a one-shot solution or as part of a model predictive control algorithm. To mitigate the growing computational complexity of the proposed algorithm, the approach is extended by proposing a distributed extension of the model predictive control formulation. Finally, a simulation based evaluation of the various formulations is conducted, and the salient features of the formulation are highlighted.

Cost-effective retrofit technology for reducing peak power demand in small and medium commercial buildings

This article describes a cost-effective retrofit technology that uses collective control of multiple rooftop air-conditioning units to reduce the peak power consumption of small and medium commercial buildings. The proposed control uses a model of the building and air-conditioning units to select an operating schedule for the air-conditioning units that maintains a temperature set-point subject to a constraint on the number of units that may operate simultaneously. A prototype of this new control system was built and deployed in a large gymnasium to coordinate four rooftop air-conditioning units. Based on data collected while operating this prototype, it is estimated that the cost savings achieved by reducing peak power consumption is sufficient to repay the cost of the prototype within a year.

Development and experimental demonstration of a plug-and-play multiple RTU coordination control algorithm for small/medium commercial buildings

There have been very few advanced control algorithms developed and deployed for small/medium commercial buildings due to practical difficulties such as the lack of Building Management System (BMS), significant disturbances, high sensor costs and high cost of site-specific engineering solutions. These high implementation costs have been a major impediment to successful market penetration. The focus of this work is to develop and demonstrate a practical control algorithm for the coordination of multiple roof top units (RTUs). The goal is to minimize sensor and configuration requirements in order to enable a more cost effective control implementation for small/medium commercial building applications. The algorithm characterizes the responses of the thermostat sensors to RTU cycling and uses a model within an optimization algorithm to determine the best combination of RTUs to operate in order to minimize power and maintain comfort conditions over a short prediction horizon. The algorithm was first tested in simulation for an existing sit-down restaurant and then at a field site that is a gymnasium. The estimated energy savings were greater than 20% for the restaurant and about 8% for the gym. Peak demand savings of about 40% were demonstrated for the gym. The algorithm is termed “plug-and-play” because no additional sensors are needed for implementation and the thermostat response models and optimization require minimal configuration.

Development, validation and application of a coupled reduced-order CFD model for building control applications

Small-to-moderate sized commercial buildings commonly use rooftop units (RTUs) to provide indoor comfort. These applications are often characterized by significant spatial variations in comfort due to poor thermostat placement and poor coordination of RTUs leading to high energy and demand costs and marginal comfort. Tools are needed that can assess both energy and comfort performance for these types of applications so that the benefits of improved RTU coordination and control can be evaluated in terms of energy and comfort and so that appropriate real-time strategies can be developed. In this paper we discuss a method for generating and coupling tractable reduced order models for the building envelope and for the indoor air dynamics. Overall assessments of the coupled model are performed for a typical sit down restaurant that employs four rooftop units. Measurements are available from the site to validate the models.

Rooftop air-conditioning unit performance improvement using refrigerant circuitry optimization

This study demonstrates the performance improvement of an air-to-air 7.5 Ton (26.4 kW) rooftop unit (RTU) achieved by optimizing an evaporator's refrigerant circuitry using evolutionary algorithms. We measured its cooling capacity and Coefficient of Performance (COP) in an environmental chamber where we also measured the in-situ air velocity profile using Particle Image Velocimetry (PIV).We used a detailed heat exchanger model to simulate the performance of the original evaporator operating with the measured air distribution, and then we used an evolutionary algorithm-based optimization module embedded in the simulation tool to optimize the refrigerant circuitry for the measured inlet air distribution.The optimized refrigerant circuitry design was implemented in a new prototype evaporator, which replaced the original evaporator in the tested RTU system. Laboratory tests of the new system showed an improvement of 2.2 ± 1.5% in capacity and 2.9 ± 1.5% in COP compared to the performance of the original system, which was optimized by the manufacturer using conventional methods prior to the onset of this study.

Development of a variable speed compressor power model for single-stage packaged DX rooftop units

Variable speed technology was applied to compressors for rooftop units (RTUs) and heat pumps in recent years. Compressor power is a critical parameter to evaluate system performance and conduct fault detection and diagnosis (FDD). However, existing power models often use numerous parameters related to the compressor geometrical dimensions and refrigerant physical properties which makes the existing models hard to implement in the field. In this paper, a semi-theoretical compressor power model was developed for single-stage RTUs equipped with a variable speed compressor and a variable speed indoor fan, based on the theoretical analysis and experimental studies. Under the normal conditions, the compressor power is correlated to the outside air temperature and compressor speed with a relative error of ±8%. This model can be employed in the field to develop a compressor power baseline for real-time FDD on the Direct Expansion (DX) RTUs.

Experimental study on electrical signatures of common faults for packaged DX rooftop units

Fault detection and diagnosis (FDD) is an effective way to automatically detect the existing faults of a rooftop unit (RTU) and increase the reliability and availability of system. Most existing FDD methods use thermodynamic signals as fault indicators. Recently, however, major concerns have arisen in regards to whether refrigerant-related temperature sensors and intrusive pressure measurements are reliable. Electrical signal measurements provide plenty of information about a system but require the use of an electrical meter. One way to solve this issue is to install variable frequency drives (VFDs) on the HVAC system, since VFDs not only optimize the system performance, but also provide electrical parameter readings. In this study, a series of lab experiments were conducted to investigate the relationship between the electrical signals and common faults. Since environmental influences have been correlated with the system performance, the impact of several driving conditions on the electrical signals was also investigated. This study identified an electrical signature for each fault and individually separated the common faults based on three parameters: the fan power, compressor power, and the supply air temperature. The results of the study are of use to the development and implementation of a new FDD method based on combining the electrical and temperature parameter measurements.

Acoustical considerations in design and construction of Turkish Contractors Association headquarters

A LEED candidate building of Turkish Contractors Association Headquarters is a prestige building located in Ankara, the project and construction of which is sponsored by leading contractor companies of Turkey. As it represents the status of Turkish construction industry, from concept phase to the very recent inauguration of the building, the major consideration has been the application of latest technology and concepts such as sustainability within the scheme of building that can pioneer future works in the field. Chilled beam ventilation is one example of new technologies applied in the building system design that takes into account high energy efficiency with minimum use of fuel or natural sources. In acoustical terms, the building envelope and structural members together with interior and environmental noise sources in relation to the building services are studied. In order to provide acoustical comfort levels in acoustically sensitive spaces and to control noise and vibration at the source and sound paths, materials and methods are developed. Specifically acoustical interventions and solutions proposed for multi-purpose hall, offices, board and meeting rooms, foyers, mechanical rooms, roof-top units, and generators located close by at the site are discussed within the context of this paper.

Virtual Surface Temperature Sensor for Multi-zone Commercial Buildings

Multi-zone structure is commonly used in small commercial office buildings, retail stores and supermarket. While there is no adjacent partition wall between the zones, the impact of a neighbor zone on the current zone can be approximated and analyzed through the application of virtual walls. It is critical to accurately estimate the virtual wall surface temperature in order to evaluate the model uncertainty and apply improved supervisory control on multiple rooftop air-conditioning units (RTUs). We propose an innovative virtual surface temperature sensor based on system-identification to solve this challenge. The validation of the virtual temperature model is processed by the three validation criteria: goodness of fit (G), mean squared error (MSE) and coefficient of determination (R2) through off-control conditions with data obtained from a building simulation platform. Further, the sensitivity analysis using the on-control three conditions (under-sizing, properly sizing and oversizing condition) is conducted for analyzing and evaluating the performance of this system-identification based virtual sensor.

A NEW AIR CONDITIONING SYSTEM FAN MODEL BASED ON NUMERICAL ANALYSIS

A large portion of energy use in buildings is attributed to air movement devices. Accurate estimation of fan performance is a key element in maximizing fan efficiency. This study proposes a new fan model that can be used in several applications such as optimization and fault detection and can also be incorporated into any commercial building models. The model uses a numerical analysis based on an interpolation technique for the data generated by basic fan laws. It can use any two variables among all four variables of airflow rate, total fan pressure, speed and power as inputs or outputs. Another advantage of this model is the flexibility of using any size of data for calibration, obtained either from manufacturers or field measured data. The model was tested for accuracy using two different manufacturers’ data of roof top unit packages with capacity ranging from 2 to 20 tons. Furthermore, the model was evaluated and tested on an actual VAV system using three months’ worth of measured data. The results show that the model can provide accurate estimation with the Coefficient of Variance (CV) less than 2% and it can be used for several applications.

The relationship between filter pressure drop, indoor air quality, and energy consumption in rooftop HVAC units

HVAC filters are commonly used to decrease exposure to particulate matter, yet little is known about the energy impacts and air quality consequences of high efficiency filters installed in commercial buildings. To explore these effects, system airflow, filter and coil pressure drop, fan pressure rise, and power draw were measured, and cooling capacity and compressor power were modeled for at least four filter pressure drops in 15 rooftop units equipped with and without fan speed control. Energy implications and clean-air-delivery-rate were estimated for a large dataset of filters divided into four efficiency (MERV) categories. Field measurements conducted on units without fan speed control showed that increased filter pressure drop decreased flow, cooling capacity, and power. For a unit with fan speed control, the same increase in pressure drop resulted in the same magnitude change of fan power but in the opposite direction, and other parameters were unchanged. Replacing MERV 8 with MERV 13/14 resulted in higher energy consumption (2–4%) during cooling mode for both unit types, energy savings during fan-only mode (8–13%) in units without fan speed control, and increased energy consumption in fan-only mode (11–18%) in the unit with fan speed control. Energy consumption increases were offset by improvement in clean-air-delivery-rate, especially for PM2.5 (2.9–3.8 times increase going from MERV 8 to MERV 13/14), with larger benefits achieved for the unit with fan speed control. A comprehensive understanding of the impact of filtration is essential to selecting the appropriate efficiency of filters that ensures low-energy use and a healthy indoor environment.

Thermo-economic optimization of rooftop unit’s evaporator coil for energy efficiency and thermal comfort

In this paper, the optimization-simulation approach is proposed to investigate energy saving potential of an air-cooled direct expansion rooftop package air conditioning system by refining the model of the HVAC system components and deriving optimal configuration for evaporator coil subject to technical constraints. In this method the frontal area of the evaporator coil is maintained as constant and the variation of other geometrical parameters on the thermal and economical performance of the system is investigated. An actual air-cooled rooftop package of a real-world commercial building in hot and dry climate conditions is used for experimental data collection. Both inputs and outputs are measured from the field monitoring in two summer weeks. Based on the mathematical models and using collected data, modules incorporating the proposed optimal redesign procedure were embedded in a transient simulation tool. A mixed heuristic- deterministic optimization algorithm was implemented in the transient tool to determine the synthesis and design variables that influence the cost and energy efficiency of each configuration. Available experimental results were compared to predicted results to validate the model. Afterwards, the computer model was used to predict how changes in cooling coil geometry would affect the building thermal comfort, the cost and energy consumption of the system.

Analysis of HVAC system oversizing in commercial buildings through field measurements

This study analyzes the oversizing issues of HVAC equipment in commercial buildings based on the data from long-time field measurements. Specifically, retail stores are selected as the typical commercial buildings to evaluate the status of equipment oversizing and its effect on energy consumption. Rooftop units (RTUs) in 12 retail stores located in different climatic regions are analyzed in terms of the oversizing status in both cooling and heating mode. The proposed method utilizes three parameters, namely cycling number (N), run time fraction (RTF), and maximum cycling number (Nmax) to jointly determine the performance of a RTU based on the annual design condition. The accuracy of the methodology is evaluated by self-validation in terms of uncertainty and compared with previous studies. The results can be used to evaluate the oversizing level of RTUs and quantify the average energy penalty of sample buildings. Designers can also use the findings as a reference to evaluate building load design. More importantly, the analytical process presented in this article can be automated and applied in the smart building management system for the advanced soft repair of an oversizing issue with RTUs.