Model-based Optimal Control of Variable Air Volume Terminal Box

In the U.S. A Variable Air Volume (VAV) system is one of most commonly used air system for multiple-zone commercial buildings due to its capability to meet the varying heating and cooling loads of different building thermal zones. One of key component of VAV system is the terminal VAV box. There are an air damper and a reheat coil in the box. How to effectively and efficiently control the VAV box plays a significant role to reduce energy consumption and maintain acceptable indoor environment in buildings. Currently, there are two control logics used for controlling VAV box, namely, single maximum and dual maximum control logics. The single maximum logic is the most common, where the room temperature setpoint is maintained by only adjusting the reheat coil valve position in the heating model. The damper position is kept as the minimal to satisfy the ventilation requirement only. On the other hand, the more advanced dual maximum control logic realizes the room air temperature control by adjusting both damper position and reheat coil valve position in the heating model. For the cooling model, both control logics have the same action to maintain room air temperature setpoint through adjusting the damper position. Â In this study, a model-based optimal control is explored to minimize the energy consumption of the VAV box with a hot water reheat coil. Data driven approach based on an Autoregressive exogenous (ARX) model is investigated to represent dynamics of the room thermal response. The similar data-driven approach is used to develop an energy consumption model of the VAV box. Measured data for the VAV box from a real building is used to train and test data-driven model. Such data includes room air temperature, outdoor air temperature, supply air temperature, supply air flow rate, damper position, reheat coil valve position and VAV box energy consumption. A platform of AMPL (A Modeling Language for Mathematical Programming) is used to for mathematical modeling and links to different optimization solvers. Â In addition, uncertainty analysis and sensitivity analysis are conducted to help understand the model behaviors and performance. In this study, the Monte Carlo sampling method is applied to generate samples for model inputs including supply air temperature, outdoor conditions, etc. A quantified sensitivity index of Sobol is calculated to indicate the impact level from different inputs or disturbances

Investigation on A Ground Source Heat Pump System Integrated With Renewable Sources

Buildings consumed 40% of the energy and represented 40% of the carbon emissions in the United States. This is more than any other sector of the U.S. economy, including transportation and industry. About 24% of all energy used in the nation is for space heating, cooling and water heating in buildings. Enhancing building efficiency represents one of the easiest, most immediate and most cost effective ways to reduce carbon emissions. One of energy efficient and environment friendly technologies with potentials for savings is Ground Source Heat Pump (GSHP) system. On the other hand, solar energy is considered as an unlimited and an environment friendly energy source, which has been widely used for solar thermal and solar power applications. This paper presents a laboratory test facility for a solar powered ground source heat pump system. The ultimate technical goal is to apply the solar powered ground source heat pump into a net-zero energy building, where all the electricity consumption will be covered by an integrated on-site solar Photovoltaics (PV) panels and battery system. The added-on benefits from this solar powered GSHP include but not limited to: 1) help further reduce electricity peak demand and 2) help further reduce greenhouse emissions. In this test rig, a ¾ - ton water-to-air GSHP is connected to two 60-feet deep wells. A group of solar PV panels of 1.12KW is connected to a battery bank, which is used to power the GSHP and a 0.27KW DC powered well pump. During the daytime, solar PV panels convert solar photons into electrical energy which will be stored into the battery bank. Whenever the GSHP system is on demand, the battery bank will provide the power. This test rig also has a comprehensive performance monitoring and data acquisition system. Well groundwater temperatures, refrigerant temperatures, air temperatures, water flow rates, etc. are all real-time monitored, trended and stored. In addition, an on-site weather station is installed to measure outside air temperature, relative humidity, wind speed and direction, and solar radiation. The details for the design and layout of this solar powered GSHP, together with the monitoring and data acquisition system will be introduced in this paper. In addition, the preliminary data collected from a testing of a cooling mode operation will be presented to illustrate the benefits of the proposed system. Finally, the feasibility of the application of the system will be discussed in the paper

A thermo-activated wall for load reduction and supplementary cooling with free to low-cost thermal water

A building envelope serves as a thermal barrier and plays an important role in determining the amount of energy used to achieve a comfortable indoor environment. Conventionally, it is constructed and treated as a passive component in a building thermal energy system. In this article, a novel, mini-tube capillary-network embedded and thermal-water activated building envelope is proposed to turn the passive component into active, therefore broaden the direct utilization of low-grade thermal energy in buildings. With this proposed approach, low-grade thermal water at a medium temperature close to the ambient environment can be potentially utilized to either counterbalance the thermal load or indirectly heat and cool the space. With the revealing of the idea, effects of water temperature and flow rate on the envelope’s thermal performance are investigated using a transient model. The results indicate that the thermo-activated wall can be effective in stabilizing the internal surface temperature, offsetting the heat gain, and supplying cooling energy to the space in summer. Utilization of the innovation should take the cost of total energy, energy benefit and efficiency into consideration. This article illustrates how low-grade energy can be actively used as a means for achieving net-zero energy buildings

Location and Optimization Analysis of Capillary Tube Network Embedded in Active Tuning Building Wall

In this study, a building wall with a thermal tuning function is further investigated. This design turns the building wall from a passive thermal system to an active system. A capillary tube network is installed inside the wall to manipulate the thermodynamics and realize more flexibility and potentials of the wall. This novel building wall structure performs efficiently in terms of building load reduction and supple-mentary heating and cooling, and the structure is convenient for applying low grade or natural energy with a wider temperature range. The capillary tube network\u27s location inside the wall greatly impacts the thermal and energy performance of the building wall. The effects of three locations including external, middle and internal side are analyzed. The results indicate that the internal wall surface temperature can be neutralized from the ambient environment when the embedded tubes are fed with thermal water. The wall can work with a wide range of water temperature and the optimal location of the tube network is relatively constant in different modes. Power benefit with the wall changes from 2 W to 39 W when the outdoor air temperature changes, higher in summer than in winter

A thermo-activated wall for load reduction and supplementary cooling with free to low-cost thermal water

A building envelope serves as a thermal barrier and plays an important role in determining the amount of energy used to achieve a comfortable indoor environment. Conventionally, it is constructed and treated as a passive component in a building thermal energy system. In this article, a novel, mini-tube capillary-network embedded and thermal-water activated building envelope is proposed to turn the passive component into active, therefore broaden the direct utilization of low-grade thermal energy in buildings. With this proposed approach, low-grade thermal water at a medium temperature close to the ambient environment can be potentially utilized to either counterbalance the thermal load or indirectly heat and cool the space. With the revealing of the idea, effects of water temperature and flow rate on the envelope’s thermal performance are investigated using a transient model. The results indicate that the thermo-activated wall can be effective in stabilizing the internal surface temperature, offsetting the heat gain, and supplying cooling energy to the space in summer. Utilization of the innovation should take the cost of total energy, energy benefit and efficiency into consideration. This article illustrates how low-grade energy can be actively used as a means for achieving net-zero energy buildings

Investigation of a Coupled Geothermal Cooling System with Earth Tube and Solar Chimney

We present a systematic study of a coupled geothermal cooling system with an earth-to-air heat exchanger and a solar collector enhanced solar chimney. Experiments were conducted with an existing test facility in summer to evaluate the performance of the system, in terms of passive cooling capability, active cooling capability, and soil thermal capability. Correspondingly, three different tests were carried out in 43 days in a sequence, from a passive cooling mode to an active cooling mode, and then back to a passive cooling mode. The results show that the coupled geothermal system is feasible to provide cooling to the facility in natural operation mode free without using any electricity. The solar collector enhanced solar chimney can provide more airflow to the system during the daytime with a stronger solar intensity. The thermal sensation analysis based on predicted mean vote and predicted percent of dissatisfied people indicates that the indoor air condition under the natural airflow stage was more acceptable in terms of thermal comfort than that of the forced airflow stage. The cooling capacity of the coupled system drops quickly after the one week forced airflow test due to the underground soil temperature increase. It takes the soil over two weeks to fully recover from the thermal saturation after the forced air test. In addition, the underground soil temperature test results indicate that the underground heat dissipation in the horizontal level was greater than that in the vertical level. The findings suggest that a minimum level of control on the system and consideration on soil saturation is needed to further improve the overall performance

Investigation on Soil Thermal Saturation and Recovery of an Earth to Air Heat Exchanger under Different Operation Strategies

A great portion of the primary energy is consumed by space heating and cooling in buildings. The need for utilizing more renewable energy in the building sector remains critical for ensuring the energy and environment sustainability. Geothermal energy is one of the renewable energy sources that we have an easy access to for supplying low grade thermal energy with a low impact on the environment. The methods of utilizing geothermal energy for buildings include such as ground source heat pumps and earth to air heat exchangers (EAHEs). Understanding the thermal saturation and recovery of soil around the heat exchangers is of great importance to ensure the successful and efficient use of a geothermal energy based system. This study addresses two dimensional dynamic heat transfer mechanism of EAHE through a transient control volume method. The soil computing domain is divided into control units along the axial and radius directions. A thermal balance of each unit is built to calculate the whole soil domain temperature based on a sequential method. The numerical result is validated by testing data based on an existing experimental facility. In order to analyze the self-recovery ability during the nonworking time, the performance of EAHE under both continuous and intermittent operation conditions is discussed. The research suggests that the soil temperature and the cooling capacity can recover during the nonworking time in an intermittent operation mode. The recovery capability of soil gradually reduces along the axial of the tube away from the inlet with the soil temperature increasing. The supply air temperature and the cooling capacity in the intermittent operation mode are more powerful than that of continuous operation mode. The research can be utilized for the design and operation management of EAHE

Performance of a Coupled Cooling System with Earth-to-Air Heat Exchanger and Solar Chimney

Buildings represent nearly 40 percent of total energy use in the U.S. and about 50 percent of this energy is used for heating, ventilating, and cooling the space. Conventional heating and cooling systems are having a great impact on security of energy supply and greenhouse gas emissions. Unlike conventional approach, this paper investigates an innovative passive air conditioning system coupling earth-to-air heat exchangers (EAHEs) with solar collector enhanced solar chimneys. By simultaneously utilizing geothermal and solar energy, the system can achieve great energy savings within the building sector and reduce the peak electrical demand in the summer. Experiments were conducted in a test facility in summer to evaluate the performance of such a system. During the test period, the solar chimney drove up to 0.28 m3/s (1000 m3/h) outdoor air into the space. The EAHE provided a maximum 3308 W total cooling capacity during the day time. As a 100 percent outdoor air system, the coupled system maximum cooling capacity was 2582Wthat almost covered the building design cooling load. The cooling capacities reached their peak during the day time when the solar radiation intensity was strong. The results show that the coupled system can maintain the indoor thermal environmental comfort conditions at a favorable range that complies with ASHRAE standard for thermal comfort. The findings in this research provide the foundation for design and application of the coupled system