Warning: Graphic Geothermal Content

Wednesday, June 20, 2012

In our last major update we introduced the Zone Load and Operating Profile Graph (the coolest graph ever). The Graph is a great tool for showing potential customers how their system is expected to operate after being installed and we thought it might be worth the time to walk everyone through the Graph, piece by piece, because there is a LOT going on.

We'll make an example out of a system that uses dual capacity heat pumps with supplemental and dual fuel operation in heating mode as our example. In order to show all possible lines, bars and background colors on a single graph, we fudged the numbers, so don't worry if your design doesn't result in a graph that matches ours. Each graph will be different but you can take what you learn here and apply it to any possible Zone Load and Operating Profile Graph.

The output graph from LoopLink will look something like this:

For clarity, we are just going to strip out all other information and discuss each component of the graph individually.

  • The x-axis (horizontal) represents outdoor air temperatures in °F.
  • The vertical y-axes use two separate scales.
    • On the left, we have Annual Bin Hours in hours.
    • On the right, we have Building Load/Heat Pump Capacity in
      thousands of Btu/hr.

Bin Data Hours

The first bit of information the graph gives, is the bar graph depicting Annual Bin Hours.

This portion of the graph shows the Annual Bin Hours (left y-axis) or the number of hours the outdoor air temperature will fall within a specific 5°F 'bin'. For example for the location shown in this graph, the outside air temperature will be between 30°F and 35°F for just under 800 of the total 8760 hours of the year.

Load Profile

Load Lines show the rate of energy addition/rejection required to maintain the building at the heating/cooling set point temperatures given an outdoor air temperature.

The y-axis on the right shows the scale for the load line in thousands of Btu/hr. There are two load lines, the red line on the left is the heating load line and the blue line on the right is the cooling load line. If you look at the lines you'll see that on the heating side of things, the rate at which energy must be added to maintain a set point temperature decreases as temperature increases and the exact opposite occurs for cooling.

Operating Profile

The next set of lines are the dashed lines representing the Operating Profile of the chosen equipment. Operating lines indicate the rate at which the equipment will be able to supply energy to the space at a specific outdoor air temperature.

This graph is for dual capacity equipment so there are two operating lines in heating and two operating lines in cooling. The lower line on both sides (longer dashes) is for Low Capacity Operation and the upper is for High Capacity Operation.

Notice how the operating lines slope in the opposite direction from the load lines. This makes sense when you think about it. Staying with the heating side of things, the load line has a negative slope because of the simple fact that the space loses heat energy more slowly as the outdoor air temperature approaches the inside thermostat set point.

At the same time, when the outdoor air temperature increases, the rate at which the equipment is able to add heat to the space increases. This is because there is less load, and therefore, less stress on the loopfield which means, more favorable Entering Water Temperatures for the heat pump.

On their own, the operating and load lines don't tell us a whole lot but when we look at them together, they start to tell a story.

Operating Profile Intersects Load Profile

The intersection of the load lines with the operating lines show where our equipment operation mode will change.

For simplicity we'll only talk about the heating side of things, just know cooling follows the same basic idea. When you look at the graph you see that the Low Capacity Heating Operation line intersects the heating Load Line at around 23°F. This implies that the equipment will be able to heat the building while operating in low capacity until the outdoor air temperature reaches 23°F. At this intersection, the equipment will need to switch into High Capacity to maintain the thermostat set point.

When the heat pump 'changes gears' to operate in High Capacity, it will satisfy the space needs until the High Capacity Operating line intersects the Load Line. At this intersection, the system needs some assistance to satisfy the space needs so we turn on supplemental (or dual fuel depending on the system design.)

Modes of Operation

The final bit of information that can be seen in the Zone Load and Operating Profile graph is the Mode of Operation at a given temperature which is indicated by the background colors.

Looking at where the High Capacity Heating Operation line intersects the Heating Load Line we see there is a background color change. The lighter tan color on the right side of this intersection shows pure GSHP operation. The slightly darker tan color to the left of the intersection is the range of temperatures where the supplemental system (typically a resistance strip heater) is operating because the GSHP can not satisfy the needs of the space by itself. The left most color is an even darker tan that indicates where the GSHP system is no longer in operation and a secondary, Dual Fuel system like a natural gas furnace has been brought online.

On the cooling side of things, the light blue background is the region where the heat pump is able to meet the demand and keep the building at the desired temperature. The dark blue area is where the heat pump isn't able to meet the demand and so the temperature will slowly increase in the building even though the heat pump is running at full speed all the time.

This sounds like a terrible thing to show a customer but if we put the bin hours back up and you look at the complete graph again you'll see that there are only a few hours out of the entire year where this would happen. If you have a lot of time in this range, you should reevaluate your equipment selections.

Understanding Time

For many people the hardest part about this graph is keeping the time component in mind. With the x-axis showing outdoor air temperature, it is easy to fall into the trap of looking at the width of the background colors as somehow indicative of the amount of time the system will operate in GSHP only, or with supplemental or when Dual Fuel is running.

Time is important because the amount of it spent operating in low capacity versus high capacity or with supplemental versus with dual fuel dictates the cost of operating the system. Just remember to focus on the number and height of the green bars within each background color and not the width of the background color.

In the example we have been working through, you can see that the majority of time (green bars) are in front of the light tan background color implying that for most of the year, the needs of the house will be met by the geothermal system all by itself. As an eyeball estimate you can see that over 5000 of the 8760 total hours of the year occur in the GSHP only heating mode area and of those, roughly 4200 hours are spent in Low Capacity operation and only 800 or so are spent in High Capacity operation.

For the 4200 hours of part-load operation during the year, it’s important to note that the GSHP won’t run 100% during that time. It will cycle on and off to meet the space needs as controlled by the thermostat. The same applies to full-load operation. LoopLink uses the bin analysis to determine the number of hours that the GSHP will actually run for each mode of operation.