In Part 1 of the Hotel Solar Thermal Study series, we looked at the main impetuses for the study: rising fossil fuel prices and a lack of hotel hot water consumption data. Part 2 focuses on the study’s methodology. How does one go about measuring hot water consumption? How are the data analyzed? How much energy do hotels use to heat water?
Measuring Hot Water and Energy Consumption
To measure hot water consumption, the only measurement needed is the flow rate of water entering (or exiting) the water heating system. In this case, the water heating system comprises three 119-gallon propane-fueled water heaters. To measure the rate of hot water consumption, the flow should be measured in the cold water supply pipe prior to its branching to the individual heaters.
I didn’t want to waste time getting approval to cut the pipe and insert a flow meter, so I used what’s called a transit time ultrasonic flow meter. This type of flow meter simply clamps on to the outside of a pipe. It sends an ultrasonic pulse through the fluid and measures the time it takes the pulse to bounce off the pipe wall and return to the receiver. The time the pulse takes to return is correlated linearly with the velocity of the fluid. Here’s a picture of the flow meter attached to a hot water pipe:
If all I wanted to know were how much hot water is consumed by the hotel, this would be the only data I’d need. But I also wanted to know how much energy was used to heat the water. In order to determine the heat input, I needed to know the flow rate of the water and the temperature of the water both before and after the water heaters added heat to it. So I measured the temperature of the water at the water heater inlet and at the outlet. To measure the temperature, I laid a thermistor flat against the outside of the pipe, then covered it with insulative foam. Because copper transfers heat so quickly, this technique is adequate for measuring the temperature of the water flowing through the pipe. Here’s a picture of the temperature sensor mounting:
All of the sensors were logged using a HOBO Microstation data logger, like this one:
Analyzing the Data
By measuring the flow rate and temperatures, I was able to determine how many BTUs of heat were added to the water each day. From there, I calculated how many gallons of propane were used, and how much this costs the hotel annually.
The heat content of propane is 91,333 BTU/gallon. The price of propane in 2011 has so far averaged around $2.52/gallon. The hotel uses an average of 572,500 BTU per day, and 209,000,000 BTU annually. If the water heaters are 95% efficient, the hotel is using 2,408 gallons of propane per year, at a cost of $6,070.
On average, this hotel used 1,300 gallons of hot water (126 degrees F) each day. The graph below shows the hot water draw profile (i.e., the hot water usage as a function of time of day). It was created by determining the profile for each day of a two-week period, then averaging the profiles to make a composite profile.
At this hotel, there is almost no hot water consumption between midnight and 5:00 am, then there’s a large usage peak centered around 8:00 am, a smaller peak around 9:30 pm, and cycles of laundry in between.
In Part 3, we’ll discuss solar thermal water heating system finances. Then we’ll determine the optimal design for a solar thermal system for this hotel. Finally, we’ll determine the effect of altering the hot water draw profile on solar thermal system performance.