Thermoelectric Heat Pump Clothes Dryer using Secondary Loop Heat Exchangers: Experimental Evaluation and System Modeling

Past work has shown that thermoelectric clothes dryers are capable of much higher efficiency than electric resistance clothes dryers. In an effort to improve performance and reduce material utilization, this work explores a new secondary loop system configuration. In this configuration, heat is transferred between air and the thermoelectric heat pumps via two water loops and two water-to-air fin-tube type heat exchangers. Performance is investigated and analyzed using both experimental and thermodynamic simulation methods

Analysis of Residential Time-of-Use Utility Rate Structures and Economic Implications for Thermal Energy Storage

Thermal energy storage (TES) is an increasingly popular tool to level out the daily electrical demand and add stability to the electrical grid as more intermittent renewable energy sources are installed. TES systems can locally decouple high thermal loads from the operation of a heat pump or reduce the electrical energy demand of the heat pump by providing a more favorable temperature gradient. In addition, many policy makers and utility providers have introduced time-of-use (TOU) rate schedules for residential customers to better reflect the price of electricity generation and demand for specific times. TOU rate schedules price grid-provided electricity differently throughout the day depending on the region’s climate, time of year, and electrical production portfolio. Large differences between on-peak and off-peak electrical prices may create an economic advantage for a residential customer to install a TES system. In this work, the economic and energy savings are calculated for a modeled 223 square foot residential building with water/ice-based TES using a TOU rate structure. The weather data is from Fresno County, CA, ASHRAE climate zone 3B, and a representative residential TOU utility rate structure from a utility provider in California was used. The simulation was carried out for cooling only during a week of extreme hot summer daytime temperature and the results showed that total energy consumption could be reduced by 14.5% with an 87.5% reduction in on-peak energy usage when the TES is installed. The cost of operating this system for space cooling was reduced by nearly 20% using the sample utility rate plan

Review of Inorganic Salt Hydrates with Phase Change Temperature in Range of 5 to 60°C and Material Cost Comparison with Common Waxes

Phase change materials (PCMs) with desirable phase change temperatures can be used to provide a constant temperature source or sink for diverse applications. As such, incorporating PCMs into building materials, equipment, or appliances can shift and/or reduce the energy load. The motivation of this work is to identify low-cost inorganic salt hydrate PCMs that can complement current building systems and designs. Two key challenges to incorporating PCMs into building materials are 1) maintaining desirable thermal properties at large scales, and 2) developing cost-effective systems that are easily incorporated into existing structures and systems. In this work, we present an analysis of inorganic salt hydrates with phase change temperatures in the range of 5-60°C, targeted towards both space heating and cooling. The properties of the salt hydrates are compared with common waxes over the same temperature range. The results showed that salt hydrate systems such as sodium thiosulfate pentahydrate, NaS2O3·5H2O, has a latent heat as high as 201 kJ/kg at a phase change temperature of 48°C which is comparable to some paraffin waxes (213 kJ/kg at 52.5°C). At a density of 1.73 g/cm3, sodium thiosulfate pentahydrate has an energy density of 347 J/cm3 (paraffin waxes, 170 J/cm3). Moreover, it was found that salt hydrates are generally less costly per unit energy in contrast to common waxes with typical salt hydrate costs in the range of 0.001-0.01 $/kJ. This analysis shows the potential of salt hydrate PCMs for developing low-cost heating and cooling thermal energy storage systems to meet a range of applications

Model-Based Air Flow Path Optimization for Heat Pump Clothes Dryer

A heat pump clothes dryer (HPCD) is an innovative appliance that uses a vapor compression system to dry clothes. Air circulates in a closed loop through the drum, so no vent is required. The condenser heats air to evaporate moisture out of the clothes, and the evaporator condenses water out of the air stream. As a result, the HPCD can achieve 50% energy savings compared to a conventional electric resistance dryer. We developed a physics-based, quasi-steady-state HPCD system model with detailed heat exchanger and compressor models. The system model is able to simulate the inherently transient HPCD drying process, to size components, and to reveal trends in key variables (e.g. compressor discharge temperature, power consumption, required drying time, etc.) The system model was calibrated using experimental data on a prototype HPCD. Air leakages, in and out, along the closed air circulation path of HPCD cause varied effects on the performance. Understanding the location, magnitude, and direction of air leakage of the heat pump clothes dryer is critical for accurately characterizing the performance and developing a high-performance design. The system model was used to reveal the impacts. In addition, model-based parametric optimizations were conducted to design the HPCD air path and leak points for optimum performance

A Preliminary Study on Innovative Absorption Systems that Utilize Low-Temperature Geothermal Energy for Air-Conditioning Buildings

Air conditioning (A/C) systems driven by renewable energy have been studied extensively during the past decade as promising alternatives to conventional electricity-driven vapor compression A/C to alleviate stress on the grid as well as reduce CO2 emissions. Among the possible renewable energy sources to drive A/C systems, low-temperature geothermal heat ( \u3c 150°C/300°F) is quite underdeveloped despite its abundance in the United States and the unique advantage of steady output regardless of the weather compared to other renewable energy sources. A major barrier to wider utilization is the typically long distances between geothermal sources and potential end uses. In order to overcome this barrier, an innovative two-step geothermal absorption (TSGA) system was studied. With this system, the low-temperature geothermal energy is stored and transported at ambient temperature with an energy density of 360 kJ of cooling energy per kg of shipped LiBr/H2O solution (about three times higher than hot water for typical space heating applications). Key design parameters of a 900 ton TSGA chiller have been determined based on computer simulations with ORNL’s SorpSim software. A case study for applying the TSGA system at a large office building in Houston, TX indicates that, for a 10-mile distance from the geothermal site to the building, the simple payback of the TSGA system is 11 years compared with a conventional electric-driven chiller. To further improve the density of the transported energy, thereby reducing transportation cost and improving payback, a new system using 3-phase-sorption technology is proposed. In this system crystallized salt solution is used to boost the transported energy density. A preliminary study of this new system shows that the enhanced energy density has potential to significantly improve payback

Self-powered Heating: Efficiency Analysis

Conventional fuel-fired heating devices such as furnaces, boilers, and water heaters have fuel efficiency less than 100% on the basis of higher heating value. They also require electricity from the electric grid to power parasitic loads such as blowers, pumps, fans, and ignitors. The primary energy efficiency of the device accounts for both fuel used on-site and primary energy used off-site to produce electric power used by the device. This work compares conventional fuel-fired heating devices to two types of self-powered devices. A self-powered device (SPD) integrates a power cycle onboard to eliminate consumption of grid electricity. We assume that all heat rejected by the onboard power cycle is added to the process fluid, so that, compared with a conventional device, the same amount of heat is provided to the process fluid and the same amount of fuel is consumed, but grid electricity consumption is eliminated. The first SPD type is the basic one: exactly the electricity required is generated. The second type considered is the SPD with heat pump (SPD-HP), in which the power cycle generates more electricity than needed for parasitic loads, and the excess electricity is used to power a heat pump. The heat pump extracts additional heat from the ambient to boost efficiency. Both SPD and SPD-HP self-consume all the generated electricity, in contrast to combined heat and power (CHP) systems that export electricity. In this work, equations are derived to express the efficiency of three classes of heating devices: conventional (consuming grid electricity), self-powered (consuming no grid electricity), and self-powered with heat pump. The efficiency of each is derived as a function of up to six factors: (1) the fraction of combustion heat captured, (2) the rate of parasitic power consumption, (3) the fraction of electric energy dissipated as useful heat, (4) the power cycle conversion efficiency, (5) the grid efficiency, when applicable, and (6) the heat pump COP, when applicable. Scenarios are identified in which it is possible to achieve efficiency greater than 100% on a higher heating value basis. Plausible configurations using existing technology options are outlined

Experimental Measurements of Clothes Dryer Drum Heat and Mass Transfer Effectiveness

Accurate system modeling of a clothes dryer requires a drum component that displays correct trends with respect to changing conditions. In this work, a model of drum heat and mass transfer effectiveness is adopted. Within this framework, experimental measurements of drum effectiveness are investigated with respect to several variables: drum volume, load mass, cloth type, drum volumetric air flow rate, and drum entering air temperature. These data can inform the modeling and simulation of any clothes dryer with horizontal-axis, axial-flow tumble-type clothes dryer drum

Domestic Dishwasher Simulated Energy Efficiency Evaluation Using Thermoelectric Heat Pump for Water Heating and Dish Drying

A quasi-steady state, coupled thermoelectric and heat transfer model heat and mass transfer lumped model was developed to predict the energy consumption and drying performance of domestic dishwashers. A numerical finite element solution was applied, assuming that the following components could each be treated as a lumped thermal capacitance: dish load, tub, wash water, and air in tub. The model was used to predict the energy consumption savings of heating water using a thermoelectric heat pump that extracts heat from the surrounding air, and the drying performance of circulating tub air through the cold and then hot side of TE modules

Review of Low-Cost Organic and Inorganic Phase Change Materials with Phase Change Temperature between 0°C and 65°C

Phase change materials (PCMs) that undergo a phase transition may be used to provide a nearly isothermal latent heat storage at the phase change temperature. This work reports the energy storage material cost ($/kWh) of various PCMs with phase change between 0 – 65°C. Four PCM classes are analyzed for their potential use in building systems: 1) inorganic salt hydrates, 2) organic fatty acids, 3) organic fatty alcohols, and 4) organic paraffin waxes. Many salt hydrates have low material costs (0.09 – 2.53$/kg), high latent heat of fusion (100 – 290 J/g), and high densities (1.3 – 2.6 g/cm3), leading to favorable volumetric storage density and low energy storage costs, 50 – 130 kWh/m3 and 0.90 – 40 $/kWh, respectively. Some salts are notably more expensive due to their scarcity or pressures from competing industries such as lithium-based salts. Fatty acids have the lowest energy storage cost in the temperature range 8 – 17°C at 6.50 – 40$/kWh. Despite favorable latent heat (125 – 250 J/g) their low density gives (0.9 g/cm3) gives poor volumetric storage capacity, 32 – 80 kWh/m3. Fatty alcohols generally have high material costs 2.50 – 200 $/kg which leads to high energy storage costs, 40 – 3000$/kWh. With latent heat and density similar to fatty acids, fatty alcohols have poor volumetric energy storage, 43 – 55 kWh/m3. Paraffin waxes containing only a single length carbon chain have a higher energy cost (15 – 500 $/kWh) than generic paraffin waxes containing many lengths of carbon chains (7 – 30$/kWh). Pure waxes have a discrete phase change temperature due to their homogeneity. In contrast, a less refined generic wax with several carbon chain lengths is more likely to have a pronounced temperature glide during its phase change. Pure single carbon chain waxes are generally required for applications \u3c45°C as generic paraffin waxes melt between 45 – 70°C. For many waxes, a solid-solid transition occurs at temperatures below the solid-liquid phase change. For pure paraffins with carbon content ≥22 C atoms, these transitions may appear near the same temperature resembling a temperature glide. The challenges with fatty acids, fatty alcohols, and waxes are low thermal conductivity, low density, some flammability concerns, and compatibility issues with some common engineering materials such as polymers. Challenges with salt hydrates are pronounced supercooling (\u3e5°C), incongruent melting, and corrosiveness. All PCMs may degrade if exposed to ambient conditions and therefore require proper sealing