A Methodology for Modeling of Hydronic Radiant Slab Heating Systems for Predictive Control and Energy Flexibility

Radiant slab heating systems are receiving considerable attention due to the multiple advantages they offer such as improved thermal comfort in buildings and suitability for other related applications in cold climates, particularly snow melting and de-icing of pavements and infrastructure. Hydronic radiant heating can utilize low temperature renewable energy heat sources. The operation of these systems can be optimized by applying predictive control and further the energy costs can be reduced by optimizing their interaction with smart grids by utilizing the flexibility in their demand profiles. However, compared to conventional air heating systems, radiant systems have several added complexities such as the slow transient heat conduction within the slab. Efficient design and operation of radiant slabs require several critical decisions on design and control variables to maintain comfortable thermal conditions in the space and achieve slab surface temperatures within the recommended range depending on the application. This thesis presents a methodology for modelling of hydronic radiant slab heating systems with significant thermal mass (a concrete layer with embedded tubes) for predictive control to utilize the energy flexibility of the building and/or infrastructure in interaction with smart grids and dynamic pricing of electricity. The modelling approaches include low-order grey box models as well as frequency domain techniques. Each approach has its own specific advantages and unique information can be obtained from each one that complement each other when optimizing system operation and designing the control strategies. Using the developed frequency domain model of the zone, key transfer functions are calculated for a case study. By means of transfer functions, the effect of different levels of thermal mass on the zone thermal response and quantification of the energy flexibility in response to grid signals is studied. The model is used to evaluate different design and operation options on a relative basis. It is shown how transfer function analysis provides insight into the building thermal dynamics without the need for simulations. A new transfer function that relates the radiant floor heat source at the bottom of the slab to the zone air temperature is introduced and its derivation is demonstrated. By means of the transfer function, the delay between the heat input of the radiant slab and zone air temperature is determined. Experiments with a full scale test room in an environmental chamber are used to validate the key design parameters obtained from the frequency domain transfer function regarding the operational strategies for energy flexibility of the thermal zone. Frequency domain techniques may also be utilized to establish the appropriate order for low-order RC models for different applications. A low-order thermal network RC model for a case study, validated with experimental measurements, is utilized to study several predictive control strategies in response to changes in the grid price signal, including short term, more reactive changes of the order of 10-15 minutes notice. An index is utilized to quantify the energy flexibility with the focus on the peak demand reduction for specific periods of time when the electricity prices are higher than usual. It is shown that the developed predictive control strategies can aid greatly in minimizing the electricity cost of the building and up to 100% reduction in peak power demand and energy consumption is achieved during the high price periods. Low-order thermal models are also utilized to study radiant slab heating systems of infrastructure that have much higher thermal mass than the radiant slabs in buildings due to structural reasons. Hydronic heating of roads and pavements surfaces to avoid ice formation has several advantages compared to traditional surface salting and other anti-skid methods which have a lot of limitations. However, the traditional method of designing such systems does not consider the thermal mass of the system and its potential for predictive control, energy flexibility and optimized performance. Finally, predictive control strategies are presented and studied to take advantage of the hydronic slab thermal mass and minimize the energy consumption and peak power demand and are validated in a full scale experiment in an environmental chamber

A study of different modeling approaches for model-based building thermal control

This thesis presents an assessment of different modeling methodologies for developing dynamic thermal models for buildings and discusses the benefits of each for model-based thermal control in buildings. The modeling section consists of two main parts: (1) the development of a detailed dynamic thermal model by means of frequency domain techniques and transfer functions (2) the development of low-order, grey-box, RC (resistance-capacitance) thermal network models with parameter optimization. The models are verified with experimental data from the Environmental Chamber (EC) test facility at Concordia University. Environmental Chamber is an experimental facility that is designed to test and calibrate building dynamic thermal models and technologies. The advantages of each of the modeling approaches for understanding the thermal behavior of the Environmental Chamber are discussed. A detailed frequency domain model is developed from the application of first principle heat transfer equations to study the thermal characteristics of the system. A detailed lumped parameter finite difference model (LPFD) is used as a tool to calculate required data for the frequency domain model that was not available through experiment. LPFD model also provides significant insight into the actual behavior of the chamber under transient conditions. Then, the creation of low-order, grey-box, RC circuit model for the Environmental Chamber is explained, as well as a methodology for optimizing the circuit parameters to find the “effective” resistances and capacitances for a defined objective which is the fit between measured and simulated air temperature. The challenges encountered while using experimental data to perform optimization for the low-order RC circuits are discussed. Such low-order models that capture the important physics of the problem are best suited to real time MPC in building automation systems in which they can be actually implemented. Finally, an analytical frequency domain model is developed for a thermal zone in an experimental facility (one of Hydro-Québec's Twin Houses in Shawinigan). The effect of different floor coverings on the thermal response of the zone is investigated by means of the frequency domain model. Also, using the frequency domain model, the effect of increasing the thermal mass and thermal conductivity of the materials used in the zone on the thermal response of the zone is investigated. The importance of studying the magnitude of the zone transfer function for effective thermal storage in the zone in an important certain frequency range is demonstrated. The key advantage of frequency domain modeling for evaluating design options without any need to perform simulation is presented

A Study of the Effect of Zone Design Parameters on Frequency Domain Transfer Functions for Radiant and Convective Systems

This paper presents a parametric study on the effect of a number of room design parameters for radiant and convective heating sources as well as solar gains. This study is performed using frequency domain modeling approach by means of which important room transfer functions are obtained and studied. Frequency domain modeling is a useful tool for analyzing building thermal dynamics as well as different design options. The phenomena affecting energy consumption inside a building such as solar gains, exterior temperature and heating/cooling sources are usually cyclic phenomena and can be modeled by means of frequency domain techniques assuming periodic conditions in the calculations. Using frequency domain techniques, the transient heat conduction inside the walls can be accurately modeled with no discretization for the thermal mass. However, there is difficulty modeling time-varying variables in the frequency domain. This is especially important in the case of convective and radiative heat transfer coefficients which are inherently non-linear elements. The coefficients are usually linearized in order to have a linear system of equation that can be presented by means of a linear thermal network[1]. In frequency domain modeling approach usually a constant value for the convective and radiative heat transfer coefficients is assumed. However, this assumption can produce significant errors when there are large differences between surfaces temperatures for example in the case of floor heating or direct gain rooms with large windows[2]. In this case, a sensitivity analysis on the magnitude of the important room transfer functions considering different values for convective and radiative heat transfer coefficients needs to be done. A room is considered with different types of heating (convective and radiative heating sources) and different levels of thermal mass on the floor. The effect of thermal mass and floor covering on the room thermal response considering different types of heating is investigated. Magnitude of the transfer functions between room air temperature and the convective heating source is a determining element in the room air temperature fluctuations considering thermal comfort aspects. Also, in the case of radiant heating, the transfer function between room air temperature and radiant heat source can be used to determine the room air temperature swings due to the floor radiant heating source. The sensitivity of the magnitude of the transfer functions versus different values of convective and radiative heat transfer coefficients is studied and compared. This study will guide future model predictive control (MPC) research by means of frequency domain techniques to make choices such as optimal thermal mass thickness for floor heating versus convective systems. It will contribute to linking design with MPC. [1] Athienitis, A.K. and O\u27Brien, W., Eds. (2015). Modelling, design and optimization of net-zero energy buildings, Solar heating and cooling, Berlin: Ernst, Wilhelm & Sohn 2015. [2] Saberi Derakhtenjani, Ali, Candanedo, Jos A., Chen, Yuxiang, Dehkordi, Vahid R., Athienitis, Andreas K. (2015), Modeling approaches for the characterization of building thermal dynamics and model-based control: a case study. ASHRAE STBE (Science and Technology for the Built Environment) Journal (21): 824-836

Energy Flexibility Assessment of a Zone with Radiant Floor Heating System by Means of Experimental Measurements

This article investigates the potential energy flexibility of a thermal zone that contains a hydronic radiant floor heating system embedded in a concrete slab. The energy flexibility of the zone is quantified from experimental measurements for a specific zone air set point change. The experiment was carried out in an experimental perimeter zone test cell (PZTC) designed to simulate the conditions of an office space near a window which has a radiant floor heating system. The PZTC is located inside a controlled environmental chamber (EC). The EC provides the desired exterior conditions. The temperature inside the PZTC is controlled with a thermostat that adjusts the heating power delivered from the hydronic pipes to the slab. It was observed that modulating the zone air temperature setpoint results in significant changes in the heating load, and thus providing a certain amount of energy flexibility. Application of the quantified energy flexibility along with applicable strategies in response to a specific price signal profile are discussed

Model Predictive Control Strategies to Activate the Energy Flexibility for Zones with Hydronic Radiant Systems

This paper presents control strategies to activate energy flexibility for zones with radiant heating systems in response to changes in electricity prices. The focus is on zones with radiant floor heating systems for which the hydronic pipes are located deep in the concrete and, therefore, there is a significant thermal lag. A perimeter zone test-room equipped with a hydronic radiant floor system in an environmental chamber is used as a case study. A low order thermal network model for the perimeter zone, validated with experimental measurements, is utilized to study various control strategies in response to changes in the electrical grid price signal, including short term (nearly reactive) changes of the order of 10–15 min notice. An index is utilized to quantify the building energy flexibility with the focus on peak demand reduction for specific periods of time when the electricity prices are higher than usual. It is shown that the developed control strategies can aid greatly in enhancing the zone energy flexibility and minimizing the cost of electricity and up to 100% reduction in peak power demand and energy consumption is attained during the high-price and peak-demand periods, while maintaining acceptable comfort conditions

Energy Flexibility Comparison of Different Control Strategies for Zones with Radiant Floor Systems

Radiant floor systems offer significant potential for studying and developing energy flexibility strategies for buildings and their interaction with smart grids. Efficient design and operation of such systems require several critical decisions on design and control variables to maintain comfortable thermal conditions in the space and floor surface temperatures within the recommended range. This study presents a comparison of different control strategies to activate energy flexibility for zones with radiant floor heating systems. The focus of this study is on the zones with radiant floor systems for which the hydronic pipes are located deep in the concrete and therefore, there is a significant thermal lag. A perimeter zone test room equipped with a hydronic radiant floor system in an environmental chamber is used as to validate the modelling methodology. Considering a typical cloudy and cold winter day, three different control strategies for radiant heating were studied based on controlling the zone air temperature, floor surface temperature, and the operative temperature. Then considering morning and evening peak demand periods, the downward and upward energy flexibility are quantified and compared with each other for the different control strategies. It is observed that for the same 2 °C increase or decrease in the setpoint, the control strategy based on the zone air temperature results in the higher flexibility for both downward and upward scenarios compared with the floor surface and operative temperature controls. The effect of increasing window to wall ratio (WWR) is also investigated. Then, also the effect of solar gains on a sunny day on energy flexibility is studied. No significant difference in the upward and downward flexibility is observed. However, the hours of zero heating load are significantly increased due to the contribution from the solar gains

Energy Flexibility Comparison of Different Control Strategies for Zones with Radiant Floor Systems

Radiant floor systems offer significant potential for studying and developing energy flexibility strategies for buildings and their interaction with smart grids. Efficient design and operation of such systems require several critical decisions on design and control variables to maintain comfortable thermal conditions in the space and floor surface temperatures within the recommended range. This study presents a comparison of different control strategies to activate energy flexibility for zones with radiant floor heating systems. The focus of this study is on the zones with radiant floor systems for which the hydronic pipes are located deep in the concrete and therefore, there is a significant thermal lag. A perimeter zone test room equipped with a hydronic radiant floor system in an environmental chamber is used as to validate the modelling methodology. Considering a typical cloudy and cold winter day, three different control strategies for radiant heating were studied based on controlling the zone air temperature, floor surface temperature, and the operative temperature. Then considering morning and evening peak demand periods, the downward and upward energy flexibility are quantified and compared with each other for the different control strategies. It is observed that for the same 2 °C increase or decrease in the setpoint, the control strategy based on the zone air temperature results in the higher flexibility for both downward and upward scenarios compared with the floor surface and operative temperature controls. The effect of increasing window to wall ratio (WWR) is also investigated. Then, also the effect of solar gains on a sunny day on energy flexibility is studied. No significant difference in the upward and downward flexibility is observed. However, the hours of zero heating load are significantly increased due to the contribution from the solar gains