WSHP System Control and Optimization 3: Alternative Heat Sinks
/By Chad Edmondson
One of the greatest advantages to WSHP systems is that they can be coupled with various types of heat sinks, giving users greater opportunity to optimize the system for lowest cost of ownership and performance based on individual needs and resources. So far in this series we’ve talked mostly about WSHPs that utilize cooling towers and boilers to add and remove BTUs when needed but the earth can also do this job. This type of WSHP system is usually referred to as a geothermal system.
If you are considering adding a heat sink to your WSHP system for extra efficiency, then existing land/water resources, owner budget and/or commitment to overall sustainability will determine which type or combination of heat sinks is best for you. If you have a suitably sized piece of land or nearby water source (e.g. a large pond), a geothermal heat sink is hard to beat in terms of cost-effective heating and cooling. In fact, the EPA ranks geothermal systems as the most cost-effective systems for heating and cooling buildings. Geothermal is also considered a renewable energy source.
Partnering with Mother Earth
If you have sufficient land with suitable soil properties, a horizontal loop can be installed in 4 to 6 ft trenches for less money. Horizontal loops require quite a bit of land, but they can be installed under parking lots or even sports fields.
A nearby lake or pond can make your partnership with Mother Earth quite a bit less expensive. Installing the loop within a lake is much cheaper than drilling or cutting into the earth. The loop pipe is weighted with concrete blocks and placed at the bottom of the lake. Temperatures may not be the same as the earth depending on water depth but heat transfer is still extremely efficient. However, the lake needs to be at least 9 ft deep and large enough to accommodate the building load. Generally speaking, a minimum of 300 sq ft per ton is required for year-round heating and cooling.
A Hybrid Solution
Clearly not ever owner has the resources (land or otherwise) to install a full capacity geothermal system. In these cases, a hybrid solution that relies on a smaller geothermal loop as a heat sink and a cooling tower to help reject BTUs in the hotter months can be an efficient and affordable alternative.
In most cases this type of hybrid solution means sizing the geothermal loop for the heating load only and adding a cooling tower sized for the difference in capacity of the design heating and design cooling loads to supplement the geothermal loop during the hottest times of the year. This is the perfect solution here in the Southeast where there is more demand for cooling than heating because it still eliminates the installation and operational expense of a boiler, while reducing the size and cost of the expensive geothermal loop piping. Time and time again, a hybrid system has shown to have the best ROI when you consider various HVAC system design types.
Remember – You Have Options!
A heat sink can make for a great “heart” of any WSHP system. It is where the system derives both its simplicity and efficiency, regardless of whether it is manmade and mechanical or natural. Choosing the right heat sink for a given application will depend on a number of factors. Fortunately, there are many options that make WSHP systems accessible to a wide range of owners.
Next up we’ll explore waterside economizer requirements for water source heat pump systems.
