In the previous chapters, three different methodologies were introduced for the optimisation of hybrid geothermal systems, all of which were ‘borefield-first’ methodologies. In this chapter, the focus will be on a hybrid-first method and on how one can simulate a hybrid system using GHEtool.
Difference between optimisation and simulation
The previous three chapters were all about the optimisation of your hybrid geothermal systems, namely, given a certain hourly building load and fixed borefield size, the load profile on the borefield was optimised for either power, energy or balance in order to maximise the geothermal share. In all three methods, the goal was to put the load on the borefield first and send whatever is left to another hybrid technology.
When simulating hybrid systems, the approach is slightly different. Instead of putting all the load on the borefield by default, the question is now ‘when is it beneficial to put extra load on the hybrid system?’ This does not necessarily yield the same result.
To illustrate this, let us consider the following example. Imagine a hybrid geothermal system consisting of a ground-source heat pump (GSHP) and an air-source heat pump (ASHP). With the optimisation strategies, the load will be put as much as possible on the borefield, only using the ASHP whenever it is necessary. However, it can be that in the mid-seasons, when the outside temperature is already rather high and the heating demand is already lower, the efficiency of the ASHP is higher than the efficiency of the GSHP. In that case, it could, for financial reasons, be interesting to put the load on the ASHP first and whatever remains on the GSHP, i.e. a hybrid-first approach.
Simulate hybrid systems method
Conceptually, simulating hybrid systems in GHEtool can be done in two different ways:
- You work with temperature thresholds above/below which a certain hybrid technology switches on.
- You limit the maximum geothermal power
Both will be briefly discussed below.
Temperature thresholds and hybrid systems
The simulation of hybrid systems in GHEtool Cloud happens based on temperature thresholds of the outside temperature. Depending on whether the outside temperature is warmer or colder than a specific threshold, it can be beneficial to use the hybrid system first instead of the geothermal one.
In the example explained above, with the GSHP and ASHP, where the ASHP is typically more efficient above a certain temperature threshold, this can be modelled in GHEtool by defining the ASHP as a hybrid heating technology that works above a threshold temperature.
Another example can occur in a renovation context. Imagine a (not-so-well-insulated) school building with high-temperature emission systems connected to a gas boiler. In order to make the heating system more sustainable, a GSHP and geothermal borefield are installed in parallel to create a hybrid geothermal system. In autumn and spring, the GSHP can heat this building perfectly, but when it gets too cold outside, the gas boiler turns on to provide the required higher temperatures. This can be modelled by using a hybrid heating system that works below a temperature threshold.
A final example could be an office building, where the air handling unit (AHU) has difficulty cooling the air using the passive temperatures from the borefield when it is too warm outside. One solution could be to equip the AHU with an active component that kicks in to provide hybrid cooling above a certain temperature threshold.
Whereas the methodology described above uses a fixed temperature threshold, it is also possible to apply this reasoning with a variable fluid temperature, for example switching to an ASHP when the fluid temperature (rather than a fixed threshold) is colder than the outside temperature.
This approach is currently being investigated and will be released in the future. Conceptually, it falls somewhere between the pure simulation methods and the optimisation strategies.
Limit geothermal power
Instead of defining the different hybrid systems and their respective temperature thresholds, one can also simply limit the geothermal power. For example, when the building has a heating demand of 100 kW, you can simply limit the geothermal power to 60 kW and see what the resulting temperature profile will be. The power that cannot be delivered by the GSHP will be shown as ‘external power’ in the results.
Example in GHEtool
In order to show how the simulation of hybrid systems works in GHEtool Cloud, let us consider, for a final time, the residential building of the previous three chapters. Below, the temperature profile of the 5×5 borefield without any hybrid system is shown.
As can be seen, both the maximum and the minimum fluid temperature limits are crossed, so a hybrid solution for both heating and cooling is required. As a first simulation, let us consider a hybrid cooling technology (for example, an ASHP) of 25 kW that turns on when the outside temperature is above 20°C, i.e. when it is really warm outside. The resulting temperature profile is shown below.
In the temperature profile above, it is clear that this 25 kW of cooling with a threshold of 20°C is enough to keep the maximum fluid temperature under 17°C. In the monthly energy balance below, it is clear that a significant part of the cooling is now handled by the hybrid system.
The next challenge is to solve the issue of the minimum fluid temperature limit. An option could be to use this same 25 kW as a hybrid technology for heating as well, where it can, for example, handle part of the heating load when the outside temperature is above 8°C. The resulting minimum fluid temperature is now only -1.79°C instead of -2.72°C, and the hybrid solution for heating takes 16.5% of the entire heating demand. The monthly distribution is again shown below.
In order to stay above the minimum threshold, it could be a good idea to add a hybrid technology below a threshold to the entire system. This can, for example, be an electric heating system filling in the peak powers. When 20 kW of such a hybrid system is added, turning on below 1°C, the following temperature profile is achieved, where the fluid now stays nicely within the limits.
In the circle graph below, the different contributions to the entire hybrid system are shown. As can be seen, one third of the entire heating demand is fulfilled by the hybrid system, and more than 50% of the building cooling demand is delivered by the hybrid system for cooling.
In the simulations above, the hybrid systems were selected together with their respective temperature thresholds. Another approach is to simply set the maximum allowed geothermal heating and cooling power. When the maximum allowed geothermal heating power is 25 kW and the maximum geothermal cooling power is 50 kW, the following temperature profile can be found.
In the energy contribution graph below, it is clear that, in order to end up with a system that fits within the limits, only 4% of the cooling and 11% of the heating load should be delivered by the hybrid system. This is significantly different from the previous results and goes to show how flexible (and inherently complex) the design of hybrid geothermal systems is.
As should be clear from the different simulations above, simulating a hybrid geothermal system is really a matter of trial and error, with no guarantee (by definition) that the optimal solution is in fact found. That is why this simulation module does not stand on its own, but should be seen as another tool in your toolbox to design the ideal hybrid geothermal system for your particular project.
Conclusion
In this chapter, the methodology to simulate hybrid geothermal systems was discussed and compared with the optimisation methods. Based on the outside air temperature, four different hybrid systems (heating and cooling, both below and above the temperature threshold) can be defined in order to mimic the behaviour of the system. Besides controlling the power using temperature thresholds, it is also possible to limit the power of the GSHP and the passive heat exchanger.
In an example, it was shown that there are many different ways to select and design a hybrid geothermal system such that the fluid temperatures match the set limits. This illustrates that, in contrast to the optimisation methods, simulating hybrid systems requires quite a lot of trial and error.
Together with the optimisation methods for power, energy and balance, the option to simulate hybrid geothermal systems completes the entire toolbox to gain insight into your hybrid systems and design them optimally.