Posts tagged how it works

What does a hot day mean for the grid?

As you may have noticed it’s now officially summer and the weather is warming up.  What you may not know is how much weather impacts energy consumption, especially during the summer.  As the temperature rises, electric grids experience more load and will eventually reach their capacity.  That’s one of the reasons we have regional transmission organizations, or RTOs.  Here in Virginia, we’re part of the largest RTO in the US called PJM.  The RTO oversees the generation and transmission of electricity in its particular region to ensure reliability. They also oversee the wholesale electricity market.

An important thing to remember is that production of electricity is not a flat price for generators.  Even though our bills in Virginia have just one rate per kWh, the utility’s cost varies depending on what time of day and the amount of demand.  Just like in Econ 101, price and demand tend to change together.  For example, here is today’s load curve for PJM:

Time of day is on x-axis and demand is the y-axis.  As the day goes on, the demand for electricity increases.  As demand increases, the utility’s cost to produce power also tends to increase.  Compare that to yesterday’s locational marginal prices (real-time prices at different parts of the grid):

Each one of those lines represents a different zone in, and they have different price points and curves.  They tend to be pretty close together, but some locations are higher or lower than others depending on demand, fuel source, and generator capacity.  It’s much more complex than the simple x per kWh rate that you see at the end of the month, but some parts of the country are moving to a dynamic pricing model with variable rates during the day.

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How it works: Chilled water

Another vital part of campus infrastructure is the production and distribution of chilled water.  Chilled water is used to dehumidify and cool buildings during the summer months.  It’s a very efficient way of cooling, often two to three times more so than a residential air conditioner.  Without further adieu, here’s how it works.

Chilled water is produced at either a central plant (like at Hollins) or at each individual building (like at Emory).  A chiller uses the same principles as your home air conditioner, except that instead of cooling the air directly it cools water.  This is done because water has a great capacity to store and absorb energy.  A chiller can be either air-cooled or water-cooled.  Water-cooled chillers tend to be much more efficient because they use water as their heat exchange medium rather than air.

Regardless of the type, chillers operate in a similar fashion.  They use the vapor compression cycle to extract heat from one medium and reject it to another (just like your air conditioner at home).  A chiller uses a refrigerant that is compressed and then put through an expansion device (causing a release of heat which cools the refrigerant) to extract heat from the chilled water loop and reject it to the outside air (air-cooled) or to a cooling tower (water-cooled).  Chilled water is typically cooled to 40-45 degrees and then distributed in a series of insulated underground pipes to buildings.  Once it gets to the building, it is distributed to cooling coils in air handlers, which distribute the cooled air the same way they do heated air.  As the chilled water circulates through the building, it extracts heat from the air and is then sent back to the central plant to be cooled again.

Chillers are notorious energy hogs.  They use between .5-1.2 kW per ton of cooling produced (a ton is unit of measurement used in cooling and refrigeration and equals 12,000 btu’s).  The central plant at Hollins has 1300 tons of cooling capacity, meaning that it can use up to 1 megawatt on a really hot day.  That’s almost half of the load for the entire campus!  That’s why we’re pushing for higher temperature set points during the summer.  Finding a balance between comfort and energy efficiency is always often a complicated endeavor, though.

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How it Works: Campus Heat

Even though I deal with our various utilities and campus infrastructure on a daily basis, for many of you in the campus community these essential items are mysterious (and often uninteresting, which I can appreciate).  As long as the lights come on, the water flows, and the heat (or air conditioning) is pumping out of those vents, most people don’t give them a second thought.  But I feel that it might be good for the campus community to get a better understanding of how our campus works so they can appreciate the complexity of it all.

This is the first part in a series called How it Works.

Most of our heat on campus comes from the central steam plant. Solid or liquid fuel is fed into what is essentially a giant furnace (called a boiler, E&H coal-fired pictured right).  Boilers, like many things these days, have become complex and expensive pieces of equipment.  Our boilers are controlled by a large computer that optimizes fuel and oxygen consumption to ensure maximum efficiency.  Once the boiler is hot, carefully treated water is boiled until it becomes steam.  Once the steam reaches an adequate pressure (here it’s about 30psi), it is fed through a series of underground steam pipes that branch off to various campus buildings.  Ideally these pipes are insulated to prevent heat loss, but many of our pipes are older and have yet to be insulated.

Once the steam reaches a building, it’s pressure is reduced and it is usually (though not always) fed into a heat exchanger where the heat is transferred to the building’s heating water loop.  By this time, the steam has condensed into water and is returned (hopefully, although leaks are common) to the central plant to be reheated back into steam.  Back at the building, the hot water loop circulates through the ducts at various points in a coil that looks fairly similar to the one in your home’s air handler.  The air handler in the building and the valve for the heating coils are controlled by a thermostat that is either set locally or controlled at the central plant.  This controls the amount of air flowing through the coils and the amount of hot water circulating through the coils.  In buildings with internal heating loads (such as computer labs), the space temperature is often warmer than needed and actually has to be cooled down by chilled water (more on chillers and chilled water next week).

Hopefully this gives you a little better picture of what goes on underground and in the walls of our campus.  It’s actually more complex than I described above (and would take a team of engineers to explain properly), but that is the basic overview of what a district steam system looks like.  Even though it sounds unnecessarily complex, it’s usually an efficient way of heating large spaces (thank you, economies of scale).  That being said, there are some highly efficient decentralized systems out there as well.  Once the infrastructure is in place, though, it’s often expensive to change over to a different system.

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