Center for the Built Environment

Extreme heat causes more weather-related deaths in the United States than any other natural hazard, and these events are projected to increase in frequency, intensity, and duration. As a result, it is critical to ensure safe thermal conditions in homes while minimizing excessive cooling energy use. In California, where the median age of homes is 45 years and nearly 40% lack mechanical cooling, this deficiency undermines one of the most essential goals of housing: to shelter people from outdoor weather of a warming planet. We aim to quantify the overheating risk in the housing sector to support the development of public policies related to maximum safe indoor thermal limits and building energy use. Using the ResStock modeling framework, we created over 52,000 building models to represent California's residential housing stock and assessed overheating risks by simulating indoor temperatures and analyzing the energy impacts of adding cooling systems.Our findings reveal significant regional disparities. Southern counties face the highest overheating risk, while coastal areas are less vulnerable due to oceanic temperatures. Inland counties, such as the Sierra Nevada region, are also less affected due to higher altitudes. Approximately 1.6 million homes will require retrofitting with cooling systems, which could increase peak electricity demand by 2%. Our sensitivity analysis showed that increasing the threshold temperature reduces the number of homes subject to overheating risk. These results significantly impact the electricity grid, emphasizing the need to consider passive cooling options like shading and cool roofs or energy-efficient cooling options like fans and evaporative coolers.

Our study uses EnergyPlus simulations to examine whole-building demand and energy end-use profiles for different design options and then uses these outputs to evaluate cost and carbon impacts of each scenario in Xendee, a modeling platform designed to “right size” and balance investments in distributed energy resources (DER).

Our results show that efficiency measures are key to meet the ambitious performance metrics for this project; however, most of the technology potential occurs for heating, ventilation and air conditioning (HVAC) or domestic hot water (DHW) loads which are a relatively small portion of a mid-rise multifamily building’s overall energy use. The most meaningful strategies to reduce or shift loads for this building include DHW load shifting, energy recovery ventilation, dynamic ventilation, and ceiling fans. Envelope strategies improve overall annual building performance but become an issue when lower heat loss increases cooling during the critical afternoon peak.

Compared to efficient, packaged air source heat pumps, a hydronic heating and cooling system (also serving DHW loads) with thermal energy storage has the best energy performance, highest load shifting capability, and best thermal resilience during outages. But because heating and cooling demands are small and hydronic systems are expensive, the net benefits of thermal energy storage are not substantial.

Sizing on-site generation and storage systems to cover the “worst case” outage conditions significantly drives up system size and cost. Even small deviations from 100% coverage (95%, or 99%) can dramatically reduce size and cost without a very meaningful change in resilience.

To decarbonize the buildings and electricity sectors at a pace consistent with state and national climate goals, buildings must become grid resources, capable of morphing load profiles to accommodate variable renewable generation, facilitate cost-effective grid decarbonization, and help ensure reliable power system operation. Simultaneously, the changing climate poses increasing risks of extreme weather events and resulting power outages that developers, designers, and building operators must plan for. This yield s new questions around the best approaches to achieve these outcomes, what it costs, who benefits, and what barriers there are to widespread adoption.We seek to answer these questions by adopting a futuristic set of grid-interactive design requirements for a mid-rise affordable housing, mixed-use development: to sustain critical loads during an outage and to consume no grid electricity for residential end-uses between 4-9pm every day. We present an approach that achieves these requirements, working to decarbonize the grid and maximize resilience for the facility and its residents while also addressing shortfalls in housing supply. Our work includes detailed building energy and economic optimization combined with construction cost analysis and carbon impacts. We find that achieving the requirements comes at a ~5% cost premium, an amount not likely to be acceptable for affordable housing under current policies and industry standards. We then generalize our design into a replicable ‘recipe’ to optimize the integrated use of distributed energy resources and explore the challenges to widespread adoption. We suggest many barriers could be overcome with policy changes that would bring benefits to under-resourced populations and society as a whole.

In high- and middle-income countries, there is a great reliance on heating, ventilation, and air conditioning systems (HVAC) to control the interior thermal environment. However, these systems are expensive to buy, maintain, and operate while being energy and environmentally intensive. Easily-accessible passive and low-energy strategies, such as fans and electrical heated blankets, address these challenges but their comparative effectiveness for providing comfort in sleep environments has not been studied. Using passive strategies in combination with low-energy strategies that elevate air movement like ceiling or pedestal fans enhances the cooling effect by three times compared to using fans alone. We extrapolated our experimental findings to estimate heating and cooling effects in two historical case studies: the 2015 Pakistan heat wave and the 2021 Texas power crisis. Passive and low-energy strategies reduced sleep-time heat or cold exposure by 69-91%. The low-energy strategies we tested require one to two orders of magnitude less energy than HVAC systems, and the passive strategies require no energy input. These strategies can also help reduce peak load surges and total energy demand in extreme temperature events. This reduces the need for utility load shedding, which can put individuals at risk of hazardous heat or cold exposure. Our results may serve as a starting point for evidence-based public health guidelines on how individuals can sleep better during heat waves and cold snaps without relying on HVAC.

This study aims to measure the impact of having visual connections to nature through windows on the cognitive performance of university students, as assessed by their final exam scores. To build upon prior research conducted in controlled laboratory and climate chamber settings, which may have a gap between findings and real-world contexts, demonstrating the positive effects of window views on occupants, this study addressed the limitations of lab-based experiments by conducting a field test in university lecture rooms with 121 students enrolled in STEM classes, taking their actual final exam. In the field test, we randomly assigned the students to either of two conditions: one with windows and one without, while monitoring indoor environmental factors. The results revealed no significant difference in cognitive performance—whether measured by scores or cognitive efficiency gauged by the time taken to complete the exam—between students in conditions with and without window views. Given the known small effect size of having windows on cognitive performance and the relatively small number of data points, we recognized that further iterations of the field tests are required to accumulate a more substantial dataset and draw more robust conclusions.

Achieving thermal comfort in buildings while maintaining energy efficiency is a critical challenge in architecture and engineering design and operation. Traditional thermal comfort metrics used in the early stages of design tend to neglect two key aspects: spatial variability of thermal conditions within buildings and the promotion of passive design strategies over active conditioning systems. This oversight leads to localized discomfort, excessive energy use, and increased vulnerability to overheating. To address these issues, we propose a novel metric called spatial Thermal Autonomy (sTA). The primary advantage of sTA is its ability to capture spatial variability in thermal conditions, offering a more comprehensive view of comfort across different building zones. Additionally, sTA supports passive design by quantifying a building's capacity to maintain comfort without active energy use. We performed a simulation case study evaluating sTA for different thermal zone sizes, passive design levels, and climate scenarios. Our findings suggest that buildings with high spatial thermal autonomy tend to use less energy, demonstrate greater thermal resilience during extreme weather or power outages, and experience fewer local discomfort problems. Optimizing building designs for spatial Thermal Autonomy encourages passive design solutions in key decisions related to building form, envelope, conditioning strategies, and HVAC system design. In buildings with reduced heating and cooling loads, this approach supports the increased adoption of local low-energy personal comfort systems, such as fans or local heating solutions, and can lead to more adaptive, resilient, and comfortable indoor environments in a changing climate.

Heatwaves are one of the most dangerous natural hazards causing more than 166,000 deaths from 1998–2017. Their frequency is increasing, and they are becoming more intense. Electric fans are an efficient, and sustainable solution to cool people. They are, for most applications, the cheapest cooling technology available. However, many national and international health guidelines actively advise people not to use them when indoor air temperatures exceed the skin temperature, approximately 35°C. We used a human energy balance model, to verify the validity of those recommendations and to determine under which environmental (air temperature, relative humidity, air speed and mean radiant temperature) and personal (metabolic rate, clothing) conditions the use of fans would be beneficial. We found that current guidelines are too restrictive. Electric fans can be used safely even if the indoor dry-bulb temperature exceeds 35°C since they significantly increase the amount of sweat that evaporates from the skin. The use of elevated air speeds (0.8m/s) increases the critical operative temperature at which heat strain is expected to occur by an average of 1.4°C for relative humidity values above 22%. We also analysed the most extreme weather events from 1990 to 2014 recorded in the 115 most populous cities worldwide, and we determined that in 103 of them the use of fans would have been beneficial. We developed a free, open-source, and easy-to-use online tool to help researchers, building practitioners, and policymakers better understand under which conditions electric fans can be safely used to cool people.

Cleaner power production, distributed renewable generation, building-vehicle integration, hydrogen storage and associated infrastructures are promising for transformation towards a carbon-neutral community, whereas the academia provides limited information through integrated solutions, like intermittent renewable integration, hydrogen sharing network, smart operation on electrolyzer and fuel cell, seasonal hydrogen storage and advanced heat recovery. This study proposes a hybrid electricity-hydrogen sharing system in California, United States, with synergistic electric, thermal and hydrogen interactions, including low-rise houses, rooftop photovoltaic panels, hydrogen vehicles, a hydrogen station, micro and utility power grid and hydrogen pipelines. Advanced energy management strategies were proposed to enhance energy flexibility and grid stability. Besides, simulation-based optimizations on smart power flows of vehicle-to-grid interaction and electrolyzer are conducted for further seasonal grid stability and annual cost saving. The obtained results indicate that, the green renewable-to-hydrogen can effectively reduce reliance on pipelines delivered hydrogen, and the hydrogen station is effective to address security concerns of high-pressure hydrogen and improve participators’ acceptance. Microgrid peer-to-peer sharing can improve hydrogen system efficiency under idling modes. Furthermore, the integrated system can reduce the annual net hydrogen consumption in transportation from 127.0 to 1.2 kg/vehicle. The smart operation (minimum input power of electrolyzer and fuel cell at 65 and 80 kW) can reduce the maximum mean hourly grid power to 78.2 kW by 24.2% and the annual energy cost to 1228.5 $/household by 38.9%. The proposed district hydrogen-based community framework can provide cutting-edge techno-economic guidelines for carbon-neutral transition with district peer-to-peer energy sharing, zero-energy buildings, hydrogen-based transportations together with smart strategies for high energy flexibility.

Hydrogen-based (H2-based) interactive energy networks for buildings and transportations provide novel solutions for carbon-neutrality transition, regional energy flexibility and independence on fossil fuel consumption, where vehicle fuel cells are key components for H2-electricity conversion and clean power supply. However, due to the complexity in thermodynamic working environments and frequent on/off operations, the proton exchange membrane fuel cells (PEMFCs) suffer from performance degradation, depending on cabin heat balance and power requirements, and the ignorance of the degradation may lead to the performance overestimation. In order to quantify fuel cell degradation in both daily cruise and vehicle-to-grid (V2G) interactions, this study firstly proposes a two-space cabin thermal model to quantify the ambient temperature of vehicle PEMFCs and the power supply from PEMFCs to vehicle HVAC systems. Afterwards, a stack voltage model is proposed to quantify the fuel cell degradation for multiple purposes, such as daily transportation and V2G interactions. Afterwards, the two models are coupled in a community-level based building-vehicle energy network, consisting of twenty single residential buildings, rooftop PV systems, four hydrogen vehicles (HVs), a H2 station, community-served micro power grid, local main power grid, and local H2 pipelines, located in California, U.S.A. Comparative analysis with and without fuel cell degradation is conducted to study the impact of dynamic fuel cell degradation on the energy flexibility and operating cost. Furthermore, a parametrical analysis is conducted on the integrated HV quantity and the grid feed-in tariff to reach trade-off strategies between associated fuel cell degradation costs and grid import cost savings. The results indicate that, in the proposed hydrogen-based building-vehicle energy network, the total fuel cell degradation is 3.16% per vehicle within one year, where 2.50% and 0.66% are caused by daily transportation and V2G interactions, respectively. Furthermore, in the H2-based residential community, the total fuel cell degradation cost is US$6945.2, accounting for 33.4% of the total operating cost at $20770.61. The sensitivity analysis results showed that, when the HV quantity increases to twenty, the fuel cell degradation of each HV decreases to 2.50%, whereas the total fuel cell degradation cost increases to 42.8% of the total operating cost. Last but not the least, the cost saving by V2G interactions can compensate the fuel cell degradation cost when the grid feed-in tariff is reduced by 40%. Research results can provide basic modelling tools on dynamic fuel cell degradation, in respect to vehicle power supply, vehicle HVAC and V2G interactions, together with techno-economic feasibility analysis, paving path for the development of hydrogen energy for the carbon-neutrality transition.

Grocery stores are one of the most energy intensive building types, which makes targets for zero net energy (ZNE) particularly challenging. This study builds on a prior computational optimization study to identify combinations of energy conservation measures (ECMs) for an existing grocery store in San Francisco. As the climate changes, also the retrofit recommendations based on simulation results from historical-based weather files may vary. In this paper, we looked at how the optimization results change when accounting for climatechanges over the building’s service life by using future weather files. We found that the expected changes in future weather are sufficient to alter retrofit recommendations. This type of analysis is thus important to ensure that buildings designed now can continue to meet performance objectives into the future.