Comparison of thermal comfort between different heating systems and adaptation to different indoor climates in winter.

Individual heating systems, such as the air-source heat pump (ASHP) air-conditioner or floor heating (FH), are usually used by people living in the hot summer and cold winter (HSCW) zone of China to heat indoor climates in the winter. However, little research has been conducted in the HSCW zone on the thermal comfort difference between indoor climates heated by ASHP air-conditioners and those heated by floor heating, as well as how occupants adapt to different indoor climates. We conducted a comparative field experiment in ASHP-heated and FH-heated apartments in Nanjing to investigate how different types of heating systems influence the thermal sensation of occupants, and we conducted a comparative field experiment in ASHP-heated office buildings and naturally ventilated teaching buildings in Shanghai to investigate how occupants adapt to different indoor thermal environments. Indoor environmental parameters and body surface temperatures were measured using instruments, and occupants' thermal sensation, activity level, and clothing were evaluated using the questionnaire. The results show that floor heating improves thermal comfort by raising foot temperature compared to the ASHP air-conditioner, and that occupants become acclimatized to different indoor climates by adjusting neutral operative temperature. According to the findings, there is no need to overheat the indoor environment in the HSCW zone because occupants can adapt to their experienced thermal environment and it is critical to maintain warm foot temperature in the cool/cold indoor environment.

Optimization of air quality and energy consumption in the cabin of electric vehicles using system simulation.

In electric vehicles, the Heating, Ventilation and Air-Conditioning (HVAC) function is often performed by a heat pump. Heating and cooling the cabin air drains energy directly from the vehicle's battery. In addition, these vehicles may operate in environments with high level of air pollution. In the cabin, passengers are confined to a small space where particles and harmful gases can accumulate. In addition, the ventilation system must also handle the air which does not enter the cabin through blower operation. This "infiltration" is a function of the vehicle speed and allows pollution to enter the cabin without being filtered or thermally treated. The objective of the study is to optimize the competing goals of the HVAC system: achieving the best air quality while maintaining good thermal comfort, at minimum energy costs. A system simulation tool is calibrated to represent the heating and cooling of an electric car. With this model, the influence of key factors is evaluated. Depending on ambient conditions and other parameters (number of occupants, vehicle speed, etc.), the blower flow rate and recirculation ratio can be adjusted to reach the objectives. The management of the proportion of fresh and recirculated air allows to regulate the humidity and carbon dioxide levels. Optimum controls are proposed as good trade-offs to reduce the power consumption, while maintaining a safe and comfortable environment for occupants. Compared to the full fresh air mode, the driving range gains are estimated in cold (-15 °C) and hot (30 °C) scenarios at 9 and 26 km respectively.

Innovative hybrid strategy for quality improvement of Goji tea: ultrasonic-ethyl oleate pretreatment combined with heat pump drying.

BACKGROUND: Goji berries, renowned for their nutritional benefits, are traditionally dried to extend shelf life and preserve quality. However, conventional drying methods often result in uneven drying, color loss and reduced rehydration capacity. This study investigates an innovative hybrid strategy combining ultrasonic-ethyl oleate (US+AEEO) pretreatment with heat pump drying (HPD) to enhance the drying process of Goji berries. RESULTS: Fresh Goji berries underwent US+AEEO pretreatment, which significantly disrupted the waxy layer, enhancing drying efficiency and water infiltration during rehydration. Compared to freeze drying (FD), HPD combined with US+AEEO pretreatment resulted in higher retention of total polyphenol content (TPC) and total flavonoid content (TFC) in the Goji soaking soup. Specifically, the HPD-US+AEEO samples exhibited the highest TPC and TFC levels, significantly outperforming FD samples. Additionally, the DPPH and ABTS antioxidant assays demonstrated higher scavenging activities in HPD-US+AEEO samples. The rehydration kinetics revealed that HPD samples had a superior rehydration rate and final moisture content compared to FD samples. Low-field nuclear magnetic resonance and magnetic resonance imaging analyses confirmed enhanced water distribution and higher mobility in HPD-US+AEEO samples. Scanning electron microscopy indicated a more porous structure in US+AEEO-treated samples, facilitating better water absorption and functional component retention. CONCLUSION: The combination of US+AEEO pretreatment with HPD significantly improves the drying process of Goji berries, enhancing nutrient retention, color preservation and rehydration properties. This innovative drying method offers a promising solution for producing high-quality dried Goji berries, benefiting both the food industry and health-conscious consumers. © 2024 Society of Chemical Industry.

A comprehensive review of solar photovoltaic hybrid food drying systems.

Demand for all resources, especially food, water, and energy, has increased with the rapid increase in the world population. These resources are limited and not enough for the growing population of the world. Therefore, the food-energy-water nexus approach has gained much attention in recent years. Less energy and water consumption and less waste generation became the target for the more sustainable food production. Qualitative and quantitative losses in agricultural commodities growing in rural areas depend both on the structure of foods and incorrect or inadequate process applied to postharvest operations to preserve food. Therefore, drying is the most preferred food preservation method. To minimize the energy consumption during drying process, renewable energy technologies are emerging as an alternative solution. It reduces dependency of fossil fuels, adds value to agricultural products to meet the increasing demand, and minimizes qualitative and quantitative losses. In this work, theoretical and experimental studies of photovoltaic-thermal (PV/T) or photovoltaic (PV) integrated food-drying systems were comprehensively reviewed. The researches covered in this review are classified into three groups: PV module integrated, PV/T collector-assisted food-drying systems, and PV/T-assisted heat pump-drying systems.

Does it pay to develop a ground source heat pump system? Evidence from China.

The application potential and environmental benefits of ground source heat pump (GSHP) systems have become the focal points of decarbonization in the building sector. Synchronized and scientific analysis of GSHP systems' environmental and economic performance, however, remains lacking. This study analyzes the application prospects of GSHP systems via a life cycle assessment-based life cycle costing method, and considers China's actual status quo. The internal and external annual costs of a GSHP system per square meter are $ 4.05 and $ 1.37, respectively. Electricity generation and steel production are key processes to improve the environmental performance of a GSHP system further. Compared with coal-based heating, a GSHP system can mitigate 65%-95% of the environmental impact and 85% of external costs, except for the metal depletion impact which is 1.5 times higher than that of coal-based heating. In Shandong Province, promoting GSHP systems can substitute up to 69.4% of the district heating area, which implies reductions in fossil depletion, greenhouse gas emissions, human health impact, ecosystem quality impact, and external costs by up to 2.37 × 1010 kg oil eq, 1.08 × 1011 kg CO2 eq, 3.87 × 105 DALY, 1.18 × 103 Species. year, and $ 2.51 × 1010, respectively. In consideration of environmental and economic aspects, a GSHP system can exhibit benefits compared with coal-based heating after 2.34 years of operation. To improve the economic and environmental performance of GSHP systems, a series of recommendations on financial subsidies, renewable energy development, inter-regional power transmission, steel scrap utilization, and hydrogen reduction steelmaking is provided.

Newly generated and increased bound phenolic in lychee pulp during heat-pump drying detected by UPLC-ESI-triple-TOF-MS/MS.

BACKGROUND: During the thermal processing of fruit, it has been observed for phenolic compounds to either degrade, polymerize, or transfer into macromolecules. In this study, the bound and free phenolic compound composition, content, and phenolic-related enzyme activity of lychee pulp were investigated to determine whether the free phenolic had converted to bound phenolic during heat-pump drying (HPD). RESULTS: It was found that after HPD, when compared with the fresh lychee pulp (control), the content of bound phenolics of dried lychee pulp had increased by 62.69%, whereas the content of free phenolics of dried lychee pulp decreased by 22.26%. It was also found that the antioxidant activity of bound phenolics had also increased after drying. With the use of high-performance liquid chromatography-tandem mass spectrometry, it was identified that (+)-gallocatechin, protocatechuic aldehyde, isorhamnetin-3-O-rutoside, 3,4-dihydroxybenzeneacetic acid, and 4-hydroxybenzoic acid were newly generated during HPD, when compared with the control sample. After drying, the contents of gallic acid, catechin, 4-hydroxybenzoic acid, vanillin, syringic acid, and quercetin in bound phenolics had also increased, and polyphenol oxidase and peroxidase still showed enzyme activity, which could be related to the conversion of free phenolics to bound phenolics. CONCLUSION: Overall, during the thermal processing of lychee pulp, the free phenolics weres found to be converted into bound phenolics, new substances were generated, and antioxidant activity was increased. Hence, it was concluded that HPD improved the bound phenolics content of lychee pulp, thus providing theoretical support for the lychee processing industry. © 2021 Society of Chemical Industry.

A numerical evaluation of a novel recovery fresh air heat pump concept for a generic electric bus.

Since the outbreak of the worldwide COVID-19 pandemic, public transportation networks have faced unprecedented challenges and have looked for practical solutions to address the rising safety concerns. It is deemed that in confined spaces, operating heating units (and cooling) in non-re-circulation mode (i. e., all-fresh air mode) could reduce the airborne transmission of this infectious disease, by reducing the density of the pathogen and exposure time. However, this will expectedly increase the energy demand and reduce the driving range of electric buses. To tackle both the airborne transmission and energy efficiency issues, in this paper a novel recovery heat pump concept, operating in all-fresh air mode, was proposed. The novelty of this concept lies in its potential to be applied to already manufactured/in-service heat pump units as it does not require any additional components or need for redesigning the heating systems. In this concept, the cabin exhaust air is directed to pass through the evaporator of the heat pump system to recover part of the waste heat from the cabin and to improve the efficiency of the system. In this paper, a 0D/1D coupled model of a generic single-deck cabin and a heat pump system was developed in the Simulink environment of MATLAB (R2020b) software. The model was run in two different modes, namely the all-fresh air (as a baseline and a recovery heat pump concepts), and the air re-circulation mode (as a conventional heat pump concept with a 50% re-circulation ratio). The performance of these concepts was investigated to evaluate how an all-fresh air policy could affect the performance of the system, as well as the energy-saving potential of the proposed recovery concept. The performance of the system was studied under different ambient temperatures of -5 °C, 0 °C, and 5 °C, and for low and moderate occupancy levels. Results show that implementing the all-fresh air policy in the recovery and baseline concepts significantly improved the ventilation rate per person by at least 102% and at most 125%, compared to the air-re-circulating heat pump. Moreover, adopting the recovery concept reduced the power demand by at least 8% and at most 11%, compared to the baseline all-fresh air heat pump, for the selected fan and blower flow rates. The presented results in this paper along with the applicability of this concept to in-service mobile heat pumps could make it a feasible, practical, and quick trade-off solution to help the bus operators to protect people and improve the energy efficiency of their service.

Integrated system of exhaust air heat pump and advanced air distribution for energy-efficient provision of outdoor air.

A large outdoor air supply is required to control the airborne infection risk of respiratory diseases (e.g., COVID 19) but causes a high energy penalty. This study proposes a novel integrated system of the exhaust air heat pump and advanced air distribution to energy-efficiently provide outdoor air. The system energy performances are evaluated by the experimentally validated thermodynamic model of heat pump and heat removal efficiency model of advanced air distribution. Results show the exhaust air heat pump with advanced air distribution can save energy because of three mechanisms. First, the exhaust air heat pump reuses the exhaust air to reduce the condensation temperature, thereby improving the coefficient of performance. Second, advanced air distribution reduces ventilation load. Third, advanced air distribution reduces the condensation temperature and enhances the evaporation temperature, thereby improving the coefficient of performance. The exhaust air heat pump saves energy by 18%, advanced air distribution saves energy by 36%, and the integrated system of the exhaust air heat pump and advanced air distribution can save energy by 45%. As a specific application, compared with the conventional system (i.e., the outdoor air heat pump with mixing ventilation), the exhaust air heat pump with stratum ventilation saves energy by 21% - 35% under various outdoor air ratios and outdoor air temperatures. The proposed integrated system of the exhaust air heat pump and advanced air distribution contributes to the development of low-carbon and healthy buildings.

Inquiry into the Temperature Changes of Rock Massif Used in Energy Production in Relation to Season.

This research was undertaken to perform and evaluate the temperature measurement in the ground utilized as an energy source with the goal to determine whether significant temperature variations occur in the subsurface during the heating season. The research infrastructure situated on our University campus was used to assess any variations. The observations were made at the so called "Small Research Polygon" that consists of 8 monitoring boreholes (Borehole Heat Exchangers) situated around a borehole used as an energy source. During the heating season, a series of monthly measurements are made in the monitoring boreholes using a distributed temperature system (DTS). Raman back-scattered light is analysed using Optical Frequency Time Domain Reflectometry (OTDR). Our results indicate that no noticeable changes in temperature occur during the heating season. We have observed an influence of long-term variations of the atmospheric conditions up to the depth of a conventional BHE (≈100 m). The resulting uncertainty in related design input parameters (ground thermal conductivity) was evaluated by using a heat production simulation. Production data during one heating season at our research facilities were evaluated against the design of the system. It is possible to construct smaller geothermal installations with appropriate BHE design that will have a minimal impact on the temperature of the surrounding rock mass and the system performance.

Modeling and Experiments for a CO2 Ground-Source Heat Pump with Subcritical and Transcritical Operation.

CO2-based ground-source heat pumps (GSHPs) have the potential to be very environmentally friendly, since GSHPs operate with high energy efficiency, and CO2 has no ozone depletion potential (ODP) and a low global warming potential (GWP). We developed a prototype CO2 liquid-to-air GSHP to investigate its performance potential in residential applications. Further, we developed a detailed model of the system that simulates both cooling and heating operation; the model is the primary focus of this report. The model simulates both subcritical and transcritical operation since the system regularly operates near and above the critical temperature of CO2 (30.98 °C) during heating and cooling operation. The model considered both the refrigerant-side thermodynamic and transport processes in the cycle, as well as the air-side heat transfer and moisture removal. We performed cooling tests for the prototype CO2 GSHP that included those from the International Standards Organization (ISO) 13256-1 standard for liquid-to-air heat pumps, as well as extended tests at additional entering liquid temperatures (ELTs). The model predicted the measurements within 0.5 % to 6.7 % for COP, 1.0 % to 3.6 % for total capacity, and 3.3 % to 4.9 % for sensible capacity. We compared the measured cooling performance to published performance data for a commercially-available R410A GSHP and found that for ELTs below 20 °C, the CO2 GSHP has a higher cooling COP and total capacity than the R410A GSHP. At the 'standard' cooling rating condition (ELT 25 °C), the CO2 GSHP COP was 4.14 and the R410A GSHP COP was 4.43. At 'part-load' conditions (ELT 20 °C) the CO2 GSHP COP was 4.92 and the R410A GSHP COP was 4.99. In the future, the model can be used to investigate methods to improve the CO2 GSHP performance to meet or exceed that of the R410A system over a wider range of ELTs; possible studies include replacing the electronic expansion valve (EEV) with an ejector, optimizing the charge, and optimizing the heat exchanger geometry and circuiting.

Performance comparison of heat recovery systems to reduce viral contagion in indoor environments.

Strong ventilation increments are currently suggested for containing the airborne diffusion of COVID-19 in indoor environments. However, it can involve an unacceptable growing of energy consumption. Therefore, maximum care must be addressed to improve efficiency of ventilation heat recovery (VHR). For this purpose, this paper investigates the opportunity of a technical solution. Consisting in adding downstream of the most diffuse heat recuperator, a heat pump using exhaust air as a cold source. An autonomous high efficiency air handling unit (HEAHU) was modelled for a school application. By simulation a performance comparison was carried on with two alternative systems based only on an exhaust air heat pump (EAHP) or on a heat recuperator for different weather conditions. Results indicated that the milder climate strongly penalizes heat recuperator and this fact deeply influences the conclusions. HEAHU saving compared to energy consumption of only heat recuperator is between 31% and 46%. For EAHP this saving varies from 2.5% to 48%. Only with a milder climate, EAHP presents a lightly greater saving than HEAHU. Heat pump technology looks to be very performing to foster the efficiency of VHR, especially in presence of high ventilation rates.

The potential role of trans-critical CO2 heat pumps within a solar cooling system for building services: The hybridised system energy analysis by a dynamic simulation model.

The rotary desiccant wheels application in the air conditioning systems are used for the air dehumidification by means of hygroscopic layers for water vapor adsorption. Nevertheless, external heat sources are required for water desorption to close the air treatment cycle. This paper investigates on the possibility to integrate in that cycle a new component, such as the trans-critical CO2 heat pump, to reduce the contribution of external thermal sources. In so doing, the high temperature waste heat discharged by the heat pump hot sink can be fruitfully exploited. Additionally, a PV array has been added to the typical layout based on the solar collectors, in order to assure the heat pump electrical driving. The energy analysis is carried out by calculating the energy performance indicators of the whole cooling system, simulating it by a dynamic model built in the MATLAB SIMULINK environment. Specifically, an air handling unit has been properly sized to supply cooling load to a reference conference hall of 1200 m3, with changes in boundary conditions (i.e. solar radiation, daily temperature and relative humidity variations). Indeed, three different cities representing the most typical Italian climatic zones, have been considered for assessing the proposed technical option suitability.

High energy efficiency ventilation to limit COVID-19 contagion in school environments.

This study investigates the possibility to contain COVID-19 contagion in indoor environments via increasing ventilation rates obtained through high energy efficiency systems combining thermal recovery by heat exchanger and thermodynamic recovery by heat pump. The starting point of this assessment is a procedure to evaluate in naturally ventilated environments, the current infectious risk by using measurements of indoor/outdoor CO2 concentrations to calculate actual air changes per hour. The method was applied to some typical school environments in Italy. The results indicated very infectious situations with reproduction number Ro values up to exceed 13. But, the simulations assessed an extraordinary reduction of indoor viral concentration and consequently of the infection risk by a strong mechanical ventilation. High ventilation rates make facemasks effective even with use levels (from 50%) reasonable also for pupils. This way, R0 goes down the value one. As regards energy performance, the behavior of an autonomous high efficiency air handling unit (HEAHU), to be installed in an existing naturally ventilated classroom, was simulated in the monitored days. The results highlight the ability to achieve a reduction in energy consumption between 60% and 72%.

Thermodynamic Performance of a Brayton Pumped Heat Energy Storage System: Influence of Internal and External Irreversibilities.

A model for a pumped thermal energy storage system is presented. It is based on a Brayton cycle working successively as a heat pump and a heat engine. All the main irreversibility sources expected in real plants are considered: external losses arising from the heat transfer between the working fluid and the thermal reservoirs, internal losses coming from pressure decays, and losses in the turbomachinery. Temperatures considered for the numerical analysis are adequate for solid thermal reservoirs, such as a packed bed. Special emphasis is paid to the combination of parameters and variables that lead to physically acceptable configurations. Maximum values of efficiencies, including round-trip efficiency, are obtained and analyzed, and optimal design intervals are provided. Round-trip efficiencies of around 0.4, or even larger, are predicted. The analysis indicates that the physical region, where the coupled system can operate, strongly depends on the irreversibility parameters. In this way, maximum values of power output, efficiency, round-trip efficiency, and pumped heat might lay outside the physical region. In that case, the upper values are considered. The sensitivity analysis of these maxima shows that changes in the expander/turbine and the efficiencies of the compressors affect the most with respect to a selected design point. In the case of the expander, these drops are mostly due to a decrease in the area of the physical operation region.

Performance and Exergy Analyses of a Solar Assisted Heat Pump with Seasonal Heat Storage and Grey Water Heat Recovery Unit.

The main research objective of this paper was to compare exergy performance of three different heat pump (HP)-based systems and one natural gas (NG)-based system for the production of heating and cooling energy in a single-house dwelling. The study considered systems based on: 1. A NG and auxiliary cooling unit; 2. Solely HP, 3. HP with additional seasonal heat storage (SHS) and a solar thermal collector (STC); 4. HP with SHS, a STC and a grey water (GW) recovery unit. The assessment of exergy efficiencies for each case was based on the transient systems simulation program TRNSYS, which was used for the simulation of energy use for space heating and cooling of the building, sanitary hot water production, and the thermal response of the seasonal heat storage and solar thermal system. The results show that an enormous waste of exergy is observed by the system based on an NG boiler (with annual overall exergy efficiency of 0.11) in comparison to the most efficient systems, based on HP water-water with a seasonal heat storage and solar thermal collector with the efficiency of 0.47. The same system with an added GW unit exhibits lower water temperatures, resulting in the exergy efficiency of 0.43. The other three systems, based on air-, water-, and ground-water HPs, show significantly lower annual source water temperatures (10.9, 11.0, 11.0, respectively) compared to systems with SHS and SHS + GW, with temperatures of 28.8 and 19.3 K, respectively.

Home Energy Management System Incorporating Heat Pump Using Real Measured Data.

The demand for electricity has been rising significantly over the past years and it is expected to rise further in the coming years due to economic and societal development. Smart grid technology is being developed in order to meet the rising electricity requirement. In order for the smart grid to perform its full functions, the Energy Management Systems (EMSs), especially Home Energy Management Systems (HEMS) are essential. It is necessary to understand the energy demand of the loads and the energy supply either from the national grid or from renewable energy technologies. To facilitate the Demand Side Management (DSM), Heat Pumps (HP) and air conditioning systems are often utilised for heating and cooling in residential houses due to their high-efficiency power output and low CO2 emissions. This paper presents a program for a HEMS using a Particle Swarm Optimisation (PSO) algorithm. A HP is used as the load and the aim of the optimisation program is to minimise the operational cost, i.e., the cost of electricity, while maintaining end-user comfort levels. This paper also details an indoor thermal model for temperature update in the heat pump control program. Real measured data from the UK Government's Renewable Heat Premium Payment (RHPP) scheme was utilised to generate characteristic curves and equations that can represent the data. This paper compares different PSO variants with standard PSO and the unscheduled case calculated from the data for five winter days in 2019. Among all chosen algorithms, the Crossover Subswarm PSO (CSPSO) achieved an average saving of 25.61% compared with the cost calculated from the measured data with a short search time of 1576 ms for each subswarm. It is clear from this work that there is significant scope to reduce the cost of operating a HP while maintaining end user comfort levels.

Solar driven Stirling engine - chemical heat pump - absorption refrigerator hybrid system as environmental friendly energy system.

Aim of this paper is to present an alternative environmental friendly energy system consisting of solar driven Stirling Engine, chemical heat pump and absorption refrigeration system. Solar energy is the main energy source and waste heat is rejected by the Stirling engine is utilized by the chemical heat pump and absorption refrigerator. This system presents an alternative environmental friendly energy system that produces electricity, cooling and heating same time. A parametric research is conducted considering power output, energy efficiency and exergy destruction rate. Results are obtained numerically and discussed. According to the results, system operates most efficiently at the high temperature ratio of the working fluids of the Stirling engine and high collector surface temperature. Maximum power output is 9.463 kW, maximum energy efficiency of the hybrid system is 0.337. Comparing these results with Stirling engine, maximum power output of the hybrid system increases 14% and energy efficiency increases 13% for the hybrid system.

A Thermoelectric-Heat-Pump Employed Active Control Strategy for the Dynamic Cooling Ability Distribution of Liquid Cooling System for the Space Station's Main Power-Cell-Arrays.

A proper operating temperature range and an acceptable temperature uniformity are extremely essential for the efficient and safe operation of the Li-ion battery array, which is an important power source of space stations. The single-phase fluid loop is one of the effective approaches for the thermal management of the battery. Due to the limitation that once the structure of the cold plate (CP) is determined, it is difficult to adjust the cooling ability of different locations of the CP dynamically, this may lead to a large temperature difference of the battery array that is attached to the different locations of the CP. This paper presents a micro-channel CP integrated with a thermoelectric heat pump (THP) in order to achieve the dynamic adjustment of the cooling ability of different locations of the CP. The THP functions to balance the heat transfer within the CP, which transports the heat of the high-temperature region to the low-temperature region by regulating the THP current, where a better temperature uniformity of the CP can be achieved. A lumped-parameter model for the proposed system is established to examine the effects of the thermal load and electric current on the dynamic thermal characteristics. In addition, three different thermal control algorithms (basic PID, fuzzy-PID, and BP-PID) are explored to examine the CP's temperature uniformity performance by adapting the electric current of the THP. The results demonstrate that the temperature difference of the focused CP can be declined by 1.8 K with the assistance of the THP. The proposed fuzzy-PID controller and BP-PID controller present much better performances than that provided by the basic PID controller in terms of overshoot, response time, and steady state error. Such an innovative arrangement will enhance the CP's dynamic cooling ability distribution effectively, and thus improve the temperature uniformity and operating reliability of the Li-ion space battery array further.

A covert field-intervention experiment to determine how heating controls that conserve energy affect thermal comfort.

A field-intervention study was carried out in 106 households in Sweden. Without informing the householders, a retrofitted heat pump controller was twice disabled for 1 week at a time over a 4-week period during the heating season, using a single-blind cross-over design with two pseudorandomly selected groups of householders, each experiencing different conditions at any given time. Thermal comfort was assessed by observing the total number of times that householders made adjustments to their set point temperature under each condition. A within-household, repeated-measures analysis was performed to determine whether this indicator was positively or negatively affected when the secondary controller was disabled so the heat pump system operated as designed. While over 80% of households showed no effect, among those that did respond to the imposed changes, a Wilcoxon matched-pairs signed-ranks test indicates that disabling the retrofitted controller had a negative effect on thermal comfort (P < 0.05 for a 2-tail test), in that it resulted in significantly more thermostat adjustments. A concurrent increase in the variance of indoor temperature about the household mean was significant (P < 0.001) but small: The range within which indoor temperatures were maintained for 95% of the time (2.5 K) was increased by only 0.3 K (16%), indicating the sensitivity of the approach.

Net-zero Nation: HVAC and PV Systems for Residential Net-Zero Energy Buildings across the United States.

This study compared the energy performance and initial cost of photovoltaic (PV) and heating, ventilating, and air-conditioning (HVAC) equipment for a residential net-zero energy building (NZEB) in different climate zones across the United States. We used an experimentally validated building simulation model to evaluate various electrically-powered and commercially-available HVAC technologies. The HVAC accounted for 23.8 % to 72.9 % of the total building energy depending on the HVAC option and climate zone. Each HVAC configuration was paired with a PV system sized to exactly reach the net-zero energy target, so the economics were compared based on the initial PV + HVAC cost. Mechanical ventilation was considered with and without heat recovery; the heat recovery ventilator (HRV) saved a significant amount of energy in cold winter months and hot summer months, and the energy recovery ventilator (ERV) provided additional benefit for humid zones. The HRV was cost-effective in the cold northern latitudes of Chicago, Minneapolis, Helena, and Duluth, where energy savings reached 17.3 % to 19.7 %. In other climates, ventilation without recovery was more cost effective, by 1 % to 9 %, and sometimes even more energy efficient. The ERV was never the lowest cost option. A ground-source heat pump (GSHP) and an air-source heat pump (ASHP) were compared, with the GSHP providing significant energy savings, 24.3 % to 39.2 %, in heating-dominated climates (Chicago through Duluth). In warmer climates, the GSHP saved little energy or used more energy than the ASHP. The PV + HVAC cost was lower everywhere with the ASHP, though it is possible for colder climates that a carefully sized GSHP and ground loop could be cost-competitive. The energy and cost data as well as the required PV capacity could guide HVAC and PV designs for residential NZEBs in different climate zones.