Conference Agenda

Simulation model of closed air source heat pump drying system is built in Dymola platform, which was calibrated with experiment data. Simulation results after calibration are basically consistent with trend of experiment data, and meet accuracy requirements of simulation analysis. On this basis, effects of compressor displacement, expansion valve diameter and refrigerant (R134a) charge on performance of closed heat pump drying system are analyzed. Simulation results show: Increase of compressor displacement will lead to increase of compressor exhaust pressure, system power and heating capacity, and slight reduction of suction pressure. Heating COP first increases and then decreases with increase of compressor displacement, which means an optimal value. Increase of expansion valve diameter will lead to rapid reduction of compressor exhaust pressure, system power and heating capacity, as well as increase of suction pressure. With increase of expansion valve diameter, heating COP first increases and then decreases, and there is an optimal value. Increase of refrigerant charge will lead to increase of compressor exhaust pressure, suction pressure and system power. With increase of refrigerant charge, heating capacity and heating COP first increases and then decreases. There is an optimal value for both of them, but the corresponding refrigerant charge of optimal value is different. Through optimization analysis, the combination of maximum heating COP of closed heat pump dryer unit in this paper is compressor displacement of 27.8CC, expansion valve diameter of 0.87mm, and refrigerant charge of 1.374kg. In this case, the maximum heating COP is 2.544, and the corresponding heating capacity is 5395W.

In the US, industrial thermal processes accounted for 32% of greenhouse gas (GHG) emissions from all industrial sectors and 7% of total US emissions in 2016 [1]. High-temperature heat pumps (IHPs) are a prospective technology for electrifying and decarbonizing large-scale industrial heating. IHPs use electrical energy to upgrade waste or unused heat, providing significantly higher energy efficiency than other electrified heating technologies. IHPs deliver energy-efficient, emission-free process heat at technically feasible temperatures, reshaping the US industrial sector without major infrastructure changes. However, HTHP deployment in the US is limited due to the high energy price ratio between electricity and natural gas, lack of technology suppliers, and low awareness/acceptance among end-users. This study analyses different integrations of HTHPs in replacing direct-fired heaters for the industrial drying process. HTHPs recover waste heat from humid air (T=70℃, RH=80-90%) and provide hot air at 200℃ with a heating capacity of 3 MW. Three configurations of HTHPs are proposed and investigated, including the transcritical refrigerant cycle, the subcritical refrigerant cycle boosted with a steam mechanical vapor recompression process, and the subcritical refrigerant cycle boosted with an electric boiler. The technical and economic feasibility of the HTHPs is investigated based on a case study in the US chemical industry. The analysis demonstrates the integration of HTHPs is technically and economically feasible.

Liquid desiccant dehumidification is an energy-saving alternative to conventional vapor-compression dehumidification. Liquid desiccant systems can be designed in various manners, with one of the common system types consisting of air and desiccant in crossflow. The carryover of liquid desiccant droplets is unwanted as it loses material, can cause corrosion, and depending on the liquid desiccant can reduce the breathable air quality in HVAC applications. The integration of fabrics as a primary conduit for desiccant flow is common to improve performance and reduce carryover. A benchtop scale falling film crossflow liquid desiccant system was constructed to study the entrainment of liquid desiccant droplets sheared from a fabric by an air stream. Additionally, an investigation was conducted to define the critical characteristics of a fabric relevant to the carryover of liquid desiccant droplets. The desiccant “bulge” profiles pushing out of the fabric’s pores were modeled to identify the onset of carryover. Results show that the key non-dimensional parameter (Weber number) predicts no carryover for We <= 10.

As for thermal comfort relation of built environments, it is anticipated to control temperature and humidity simultaneously through the operation of heating, ventilation and air conditioning (HVAC) system, such as an air handling unit (AHU). However, presence of large latent cooling load can substantially increase the energy consumption in moisture removal for the typical cooling-reheating operation of conventional AHU. Augmenting AHU with desiccant wheel (DW) has been well received as a viable solution for dehumidification, in light of diverse options of regeneration heat source. Nevertheless, such system modification further complicates the system design and operation, as for the optimal coordination among the AHU, DW rotation and regeneration temperature and flow. To reap the true benefits of DW assisted AHU, optimal and cost-effective control development is thus critical, for which a high-fidelity dynamic simulation model is indispensable.

This study is concerned with a desiccant assisted direct-expansion air handling unit is developed as the core for a multi-functional AHU system for comprehensive building environment control. For a selected building, a 7-ton roof-top-unit (RTU) is chosen as the primary HVAC system, where the compressor speed is used to control the sensible cooling performance. A 76-inch-diameter desiccant wheel is adopted for dehumidification during the operation, where the wheel angular speed is used to control the dehumidification performance, along with potentially controllable temperature and flow rate for the regeneration airflow.

To facilitate control strategy development and assessment, a Modelica-based dynamic model is developed for the target system, which includes the models of DW, RTU, AHU and the building models. The desiccant wheel model is developed with transient modeling of the mass and heat transfer processes therein. The desiccant wheel is structured of rotational air channels, where dehumidification air channels are termed “process” and warm air channels are termed “regeneration” with opposite air flow directions. Thus, the wheel angular speed and heat source to the regeneration air flow are the key points of the dehumidification performance. The parameters of desiccant wheel are validated with the experiment data.

For building modeling, the lumped zone volume models are adopted, with the occupant number specified and tied up with the latent and sensible loads, as well as non-occupancy thermal load. The outdoor air goes through an air mixing box, a desiccant wheel, and a roof-top-unit to the lumped zone. The return air is partly through the mixing box then exhausted to outdoor, and the other part is warmed by a heat source then goes through the regeneration channels of the desiccant wheel to remove the contented water from the dehumidification process channels.

The developed Modelica model of the system plant is evaluated with simulations. A rule-based control method is proposed for the temperature and humidity regulation across different ambient and load patterns, with energy minimization consideration while satisfying the minimum ventilation requirement. The control inputs include the DW angular speed, the temperature and flow rate of the regeneration heat source, RTU compressor speed, and AHU damper. Also, a multi-objective optimal control strategy is developed under way.