Case Study: Combisystem for a Caribbean island hotel
The Caribbean island of Anguilla is only 16 miles long and 3 miles wide, but it boasts some of the most beautiful beaches in the world. The white sand, tall palm trees and clear blue water on the north side of the island are now getting new accommodations, with the construction of several new resort hotel buildings. A serious effort is under way to make these new buildings as energy-efficient and green as possible. That includes renewable and alternative heating for the domestic hot water (DHW), and spa-pools in the buildings.
The climate is mostly sunny and warm year-round, and sometimes a little too humid for comfort. So, air conditioning (A/C) is being installed throughout the hotels to manage the heat and humidity, but as efficiently as possible. Conventional energy rates are high, for example, the local electricity provided by diesel generators costs 4 to 5 times more than what we typically pay in the U.S. Propane is available for backup heat, but is also equally expensive. It makes good sense to offset conventional energy with solar energy, and to recover waste-heat as well. In these new buildings, the A/C systems includes a pair of hydronic heat exchangers that allow the heat that is normally rejected to the outdoor air to be collected and used to make hot water.
As it turns out, this is a perfect application for a “New Standard” thermal hydronic combisystem. And, such a system was designed about a year ago, and is now being installed to manage the thermal energy efficiency in the first of several hotel buildings under construction. Figure 73-1 shows a photo of the solar collectors on the day they were mounted on the roof of the first hotel building, on July 4, with the scenic Zemi Beach in the background.
A new-standard, renewable-energy combisystem design
When this project was originally presented to me last year, it came with a list of energy-efficient features requested as part of the original green concept. These included:
1. A central group of solar heat collectors that fit on the roof of a 5 story, 28 room hotel to provide heat for both the DHW and the spa-pools.
2. An easy way to connect the spa-pools later on (in the future), since they are considered optional heat loads, and may not be built or connected to the solar heating system at the start.
3. Propane boiler backup that includes two boilers for fail-safe redundancy to provide heat to both the DHW and the pools, but only as the fuel of last resort.
4. A/C waste heat recovery heat exchangers were to be included to supplement the DHW and pool heat whenever it is available, in concert with the solar and propane heat.
5. A comprehensive plumbing and control design that can be installed and duplicated in other buildings on the island with little modification.
When you read between the lines in the system described above, what you have is a hydronic combisystem that includes multiple heat sources and multiple heat loads. The heat sources include A/C waste heat recovery, and solar and boiler heat. The heat loads include a large DHW system for the kitchen, laundry, sinks and showers of a small hotel, and a couple of small (optional) spa-pools. This fits the requirements for a new-standard solar hydronic combisystem, as I have described it many times in this column.
Figure 73-2 shows a schematic piping diagram of the system concept that was developed for this installation. This diagram has been abbreviated here for clarity, but the distinctive new-standard primary loop configuration is unmistakable. This allows all the various components to be connected together using closely spaced tees, and then controlled in a standard way with very little customization.
Some unique design details
The piping diagram in Figure 73-2 shows a familiar standard piping configuration used in this installation, but certain details were modified for this particular job. Some of the most interesting design details include the following:
• Since this climate never freezes, plain water can be used as heat transfer fluid (boiler fluid) in all the hydronic equipment including the solar collectors.
• The desire for redundancy has been extended beyond the two boilers. There are also two DHW heat- delivery pumps, two primary-loop pumps and two A/C heat exchangers with pumps. This allows for one component in a pair to keep working while the other is serviced.
• Nearly all the circulator pumps are high efficiency Grundfos Alpha models that use about half the electric power of conventional circulators. They are controlled to run intermittently and turn off when ever possible.
• The actual capacity of the DHW heat accumulation tanks is around 930 gallons total. This is enough heat storage capacity to allow the solar or the A/C heat to be stockpiled for the occasional surge in hot water use that can happen in a hotel. In the final installation, there are actually three DHW tanks, each with an internal heat exchanger coil.
• The future plumbing connection for a spa-pool is seen on the diagram as a “stub-out” of two closely spaced tees. This is how any future heat load (or heat source) can be included in the heating system during installation.
• A SolarLogic Integrated Control system (SLIC) has been installed along with this plumbing design to provide integrated energy management, energy measurement, data logging, monitoring and remote control using the computer network and internet in the hotel.
• All planned future additions can be controlled with existing software and hardware in the SLIC control system, so no future additional controls will be needed. This includes the two future spa-pools, which can be included in the control system simply by activating that part of the software remotely when needed.
The installation of this hydronic combisystem is still under way at this writing . So, we have yet to start-up the system or collect any performance data from it. But, as the data becomes available, we will be glad to share it with you in the future.
This system is very similar in its basic configuration to many other systems we have discussed in past columns. But, this is a good example of how scalable and adaptable a standard combisystem configuration can be. In this system, the DHW components are made larger than most others, the pools are a bit smaller than many others, and the A/C heat exchangers essentially replace the additional solar heat collectors that we often install. There is no need for glycol antifreeze, so a main heat exchanger is eliminated which actually improves thermal efficiency. But, because the basic primary loop configuration and the temperature order around the loop are preserved, a standard control system can be plugged in without modification.
These articles are targeted toward residential and small commercial buildings smaller than ten thousand square feet. The focus is on pressurized glycol/hydronic systems since these systems can be applied in a wide variety of building geometries and orientations with few limitations. Brand names, organizations, suppliers and manufacturers are mentioned in these articles only to provide examples for illustration and discussion and do not constitute any recommendation or endorsement.
Bristol Stickney has been designing, manufacturing, repairing and installing solar hydronic heating systems for more than 30 years. He holds a Bachelor of Science in Mechanical Engineering and is a licensed mechanical contractor in New Mexico. He is the chief technical officer for SolarLogic LLC in Santa Fe, N.M., where he is involved in development of solar heating control systems and design tools for solar heating professionals. Visit www.solarlogicllc.com for more information.