Introduction to Central Plant

When people think about energy systems in a hospital, most naturally think of the central plant, with its large chillers and cooling towers, boilers, pumps, generators, and other big things that make noise. The problem is that many of the improvements that are often suggested for central plants are incremental - a more efficient chiller, a new boiler that raises combustion efficiency by a point or two, maybe a stack gas economizer. Those may be fine things to do, but we won't really decarbonize by making incremental improvements in the plant - we need to shift our thinking a bit more radically.

Boilers burn fossil fuels, producing carbon dioxide (and other pollutants), which contributes to climate change. We must eliminate combustion of fossil fuels. While there may be alternative fuels available in the future (hydrogen, perhaps?), they aren’t available now. Electric boilers? These loads are too large, and straight conversion would lead to a net increase in carbon emissions. What to do?

There is a solution, but first, let’s review the flow of heat in a hospital building.

In a traditional hospital design, chillers are used to remove heat from the building and boilers are used to add heat to the building – often at the same time. We all know this is a waste of resources, and there are a variety of strategies to reduce the simultaneous use of heating and cooling. Reducing load is an essential first step - but it won’t eliminate boilers.

To understand the problem, we need to understand the flow of heat. How does heat enter the building in the first place? Boilers, of course. We think of outdoor temperatures and sunshine, but that is really a small fraction of the total heat input. Other sources include lighting, medical equipment, refrigerators, and computers. Actually, almost all of the electrical energy we put into buildings becomes heat. We don’t think of electricity as heat, probably because we in the US use different units – kilowatt-hours (kWh) for electricity and British Thermal Units (Btu) for heat. But kWh and Btu are different units for the same thing – energy. Our standard measurement for building energy efficiency, EUI, is usually stated in terms of kBtu/sqft/year, which requires that electrical energy is expressed in kBtu (1kWh = 3413 Btu = 3.4 kBtu).

With small exceptions (energy used for exhaust fans is directly removed by the exhaust air, so does not become heat inside the building), every kWh that comes into a building becomes heat and must be removed.

People are another source of heat – we all generate about 500 Btu/hour of heat and moisture. In warm weather, ventilation also brings heat in.

All of this heat warms the air in the building and is removed by the HVAC system – either through air handling units to chillers to cooling towers or directly through exhaust. While exhaust removes heat directly, it must be balanced by the intake of outdoor air. In hot weather, ventilation is a net gain of heat, while in cool weather, ventilation is a net loss of heat.

A common cooling strategy in cool weather is the economizer cycle – using either cool outside air directly for “free cooling” or using cooling towers to make cold water for waterside “free cooling.” That’s another way that heat leaves the building.

Here’s what that looks like on an annual basis for a hospital in California:

On an annualized basis, it’s apparent that hospitals have way more heat than they need – losses through skin and even ventilation are small in the overall heat balance. That’s true even when it’s cold – a modern hospital may be heat-positive whenever the outdoor temperature is above 20-30°F.

So, why do we run boilers year-round, adding heat to an already-hot building? There are probably many answers to that, but it mostly boils down to "that's how we've always done it." This is a system that was developed in the days when energy was cheap, cooling was being added to buildings heated by steam, and the two systems coexisted.

This issue was brought to the fore by the landmark "Targeting 100!" report published by the Integrated Design Lab at University of Washington in 2012. They proposed several solutions and most of those ideas are still valid today.

Heat pumps are a primary answer to this dilemma of having too much heat where we don't want it and not enough heat where we need it. Heat pumps simply move energy from one place to another, raising the temperature in the process. Central plants based on moving energy around are much more efficient than those that burn fossil fuels to make "new" heat while running other machines to dump heat to cooling towers. This has been done in hospitals already and it needs to be the default system in all new hospitals going forward.

Moving heat from one part of the building to another isn't the whole answer, though. When it gets cold, our buildings need other sources, which may include solar thermal, thermal energy storage of various types, and maybe even using plumbing wastewater as a source for heat. We'll all have to be a bit creative in how we look for heat to harvest.

Within this chapter of the Guidebook, we discuss some of these technologies in more detail, and we invite you to engage in the discussion. Let's all stop burning fossil fuels to heat our buildings!