Recently, I have been conducting research on optimizing solar thermal water heating systems for hotels. Hotels are among the best sites for solar domestic hot water (SDHW) systems because of their large and constant hot water load (a third of the energy used in hotels is for heating water!), and because their roofs are typically large and flat. And yet, hotels have some interesting characteristics that make sizing SDHW systems a bit of a challenge.
One of these characteristics is that the hot water draw profile (that is to say, how much hot water is used at different times of day) for hotels is not as well-defined as the draw profile for residential buildings. If you don’t know how much domestic hot water (DHW) a building consumes or what the building’s largest rate of hot water consumption is, any attempt at system sizing will be fraught with guesswork. Also, for the purpose of estimating the system’s life-cycle cost, it is necessary to know the energy consumption of the existing system.
This study begins to address the lack of hotel hot water use data. I measured the DHW consumption at two different hotels, each for a period of two weeks. I then combined the data to find an average hot water draw profile for each hotel.
This study also looks at the effect that moving hot-water-using hotel operations (for example: laundry services) to different times of day can have on SDHW system optimization. For instance, it may be that a smaller system will be required if the laundry can be done at 3:00pm rather than in the morning. Computer simulations using TRNSYS will allow me to determine the optimal system size for each hotel by varying flow rate, collector type, total collector area (i.e., number of collectors), water storage tank size, and time of day of hot water usage. The optimal system is, for this study, defined as that which minimizes the life-cycle cost. The life-cycle cost takes into account the cost of the system, installation, maintenance costs, energy savings over the system’s practical lifespan, Renewable Energy Certificate (REC) sale revenue, state and federal tax credits, inflation, and fuel price escalation.
But before we get into the details of this study, let’s take a look at some common water-heating fuels and their price trends over the last 20 years. Here is a chart showing, for each of the most common water-heating fuels, the number of lodging industry buildings that use that type of fuel. All of the following data was gathered from the US Energy Information Administration website (www.eia.doe.gov). Click each chart to enlarge if you’d like a better view.
Because this research was done in North Carolina, I’ll share the price trends for that state, but these trends are similar for the vast majority of states. For a graph of the average US residential electricity price over the past decade, click here.
All of the fuels have a general upward trend, and prices have become more volatile over the past decade, but natural gas is coming down in price for now. Large shale reserve discoveries and aggressive drilling practices have significantly increased natural gas production capacity, while demand has remained about the same.
The two hotels looked at in this study currently heat their water with propane, so supplementing their DHW systems with solar may be a good way to hedge against rising fuel costs. Hotels that currently heat water with natural gas would probably benefit from solar thermal systems as well, but even if only those hotels using propane were to install solar systems, that would still be 13% of all US hotels.
In the next post, I’ll share with you how I measured hot water and energy consumption, and the results of those measurements. We’ll take a look at some of the hot water consumption trends in hotels and how much energy each hotel might be able to save by installing a supplemental solar thermal system.