Grid Stability
When Mustafa Sultan, BA ’25, MS ’26, was an undergraduate at Stanford, he developed a keen interest in energy efficiency in commercial buildings. As classmates gravitated toward artificial intelligence, or worried how AI might affect their job prospects, Sultan became fixated on another acronym…. HVAC.
About 15 million[1] rooftop heating, ventilation, and air conditioning systems are in operation in the United States, from Amazon warehouses to your local gym, and he learned that they account for 50 percent of the total electricity usage[2] for those structures.
“HVAC is the largest load in commercial buildings,” he says, and as one of the most expensive systems, “you’re going to use it until its last minute.”
As an economics major who also cares about sustainability, Sultan sensed a business opportunity. The more he learned about commercial HVAC, the more he wanted to learn about the technology.
The standard choice in the United States is quick and dirty: Traditional gas HVAC rooftop units are fast to install but extremely inefficient to operate, resulting in high energy costs. With some research, he discovered another highly efficient choice called variable refrigerant flow (VRF) that was invented in Japan back in the 1980s.
“I became obsessed with VRF. I’m like, this is the coolest technology in the world,” he says, perplexed that VRF was rare in the United States compared to Europe and Asia.
“If this is the most energy-efficient technology, why has it never succeeded in the U.S.?”
In a carpe diem moment, he decided the best way to find out was to order a unit, with a plan to retrofit a pilot customer’s IHOP restaurant.
Before the container ship had finished crossing the Pacific Ocean, he got his answer.
“So I started to get labor bids for $80,000, $100,000, and even more,” he says of the cost to install the equipment at the restaurant—far more than he spent on the HVAC unit, which he later resold on eBay.
Picture the drafty metal ducts in your local Costco or McDonalds. Most American warehouses and commercial buildings follow the same basic architecture: an HVAC unit sits on the building’s flat roof, where it connects with a metal ductwork system that channels hot or cold air throughout the indoor space. These gas rooftop units are heavy—from 1,000 to 4,000 pounds—and sit on top of a curved cement platform that helps prevent the roof from collapsing. Typically these units are powered by natural gas for heating and electricity for cooling.
In contrast, VRF runs exclusively on electricity. It is a commercial heat pump that transfers warmth from one location to another, known for its high level of thermal control. Each room or zone of the building has its own individual indoor unit on the ceiling that connects to the outdoor central condenser with copper piping.
A crew with a crane can install a traditional gas unit in a single afternoon, but retrofitting a building with VRF can take at least a month, as it involves tearing out the ceiling and old ductwork and then welding hundreds of feet of copper piping.
“We discovered the market failure firsthand. It made sense that gas rooftop units still dominate the commercial HVAC landscape because there is no cost-effective way to install VRF,” says Mustafa.
So, he set out to build an HVAC unit that delivers the best ROI. “We built what didn’t exist: a rooftop VRF heat pump system that installs just like a standard gas unit. It costs the same all-in and delivers far greater energy savings—making it the no-brainer HVAC choice.”
Rooflines and business plans
Mustafa Sultan founded Greenhill in 2024 in order to scale up commercial HVAC electrification.
“So my cofounder is my brother,” he says. “I convinced him to quit his job to work on this with me.”
Musa Sultan studied real estate development at the University of Southern California and earned his master’s in real estate finance from Cornell University. Before joining Greenhill, Musa worked in HVAC controls and engineering for Greystar, a development company that specializes in multifamily and mixed-use buildings.
“We developed this idea together,” says younger brother Mustafa, who is completing a master’s in Earth System Science at Stanford. “Now we’re doing this full time.”
Through networking at HVAC conferences from Las Vegas to Miami, the brothers connected with a manufacturing partner in December 2024, and together they bootstrapped their first prototype of the world’s first high-efficiency heat pump that is compatible with air ducts.
The result is a Greenhill VRF heat pump system that can be craned onto a building, fitting neatly on top of the existing platform and ductwork, combining the ease of gas rooftop unit installation with the efficiency of VRF technology.
Their prototype arrived in June 2025, and the next month they installed it in an IHOP in Northern California and began their first pilot—made possible by an Innovation Transfer Grant from the TomKat Center for Sustainable Energy.
“We have around 6 months of operating data,” says Sultan. Their findings show that the restaurant’s unit reduced its HVAC energy consumption by 60 percent, as well as its peak demand use.
Softer peaks
On any given day, power companies must be ready to supply customers with enough electricity for their maximum usage. Think of when every air conditioner in the city is set to full blast on the hottest afternoon of the year. Or when a factory is operating every machine at once. That’s called peak power demand.
Utilities will bill commercial customers according to both their total consumption in kilowatt-hours and their peak demand. The utility will monitor the customer’s power draw continuously and then identify the highest average demand during any 15-minute window in the entire billing month—and that spike can cost thousands of dollars, depending on the size of the warehouse or company.
A major advantage of VRF technology is that it is programmable, allowing building managers to set limits for power use in the building. VRF is also capable of turning on the HVAC system more gradually, rather than how gas units go full throttle like a lawnmower rattling to life on the warehouse roof, which can soften peak demand as well.
In the Greenhill pilot study, their technology reduced the restaurant’s peak demand by 40 percent.
“The beauty of VRF is that it won’t slam on right away. It will soft start. So you’re not spiking your demand anymore,” says Sultan. “That’s what everyone wants. Everyone wants lower utility bills.”
He has big ideas for the future. Battery packs (for charging HVAC units in the middle of the night when electricity rates are lowest). Checks mailed to customers (for enrolling in peak-reduction programs). Aggregators (for coordinating peak use across whole cities).
“When you network thousands of Greenhill VRF heat pumps, you can create a distributed fleet of flexible capacity across a city. You can ramp load down, shift load, or coordinate load—all without sacrificing comfort,” he says. “That’s the backbone of modern grid stability.”
Then he grins. He’s getting ahead of himself.
“That’s the long-term vision,” he says.
This article is part of the TomKat Center Spotlight series designed to highlight the impact and trajectory of the work of faculty and students who received funding through our Innovation Transfer Program, TomKat Solutions, and Graduate Fellowships. Stanford University does not endorse any non-Stanford entities, programs, products, or services listed in the article.
