Before we can start talking about the design and sizing of borefields in GHEtool, it is important to revisit our discussion on the temperature profiles once again and discuss the difference between the inlet, outlet and average fluid temperatures.
Temperature profiles in GHEtool
When we introduced the concept of temperature profiles back in Part 2.1, all fluid temperatures (both in monthly and hourly temperature profiles) were averages of the fluid temperature going into the borefield and coming out of it. This is the most common, historical definition of the fluid temperature and is used by other borefield design tools like Earth Energy Designer. An example of such a (monthly) temperature profile with average fluid temperatures is given below.
The reason why the average fluid temperature is so common is because of its direct link to the concept of the effective borehole thermal resistance, which was introduced in Part 2.2. To recap, the effective borehole thermal resistance is defined as the steady-state heat transfer resistance between the average borehole wall temperature (the average of the entire borehole wall) and the average fluid temperature (the average of all the fluid inside the borehole).
During the simulation, the borehole wall temperature is first calculated using the monthly (or hourly) extraction and injection loads and the g-functions (see Part 2.3). Once the borehole wall temperature is known, the effective borehole thermal resistance is calculated using the variable fluid and flow properties. Using these two results (the borehole wall temperature and the borehole resistance), the average fluid temperature can be calculated directly based on the definition of the borehole thermal resistance.
However, since the flow rate is also known (either constant or variable), the temperature difference between the borefield inlet and outlet is also known, based on the following formula:
$$\dot{Q}=\dot{m}C_p\Delta T$$
where $\dot{Q}$ is the extraction/injection power (kW), $\dot{m}$ is the mass flow rate (kg/s) through the borefield, $C_p$ is the specific heat capacity of the fluid (kJ/(kgK)), and $\Delta T$ is the temperature difference between the borefield inlet and outlet.
When the average fluid temperature is known, together with the mass flow rate, the power, and the specific heat capacity (at each month/hour), the inlet and outlet temperatures can also be calculated. This gives us the option to work with any of these three fluid temperatures in GHEtool, each of which tells a different story and will be discussed below.
Three fluid temperatures
Three fluid temperatures can be simulated in GHEtool: the average fluid temperature, the inlet fluid temperature, and the outlet fluid temperature. All three are discussed briefly below.
the legislation when it comes to the design of closed geothermal systems is sometimes defined with the average or the inlet fluid temperature. In Belgium (Flanders and Brussels) as well as The Netherlands, the lowest fluid temperature is defined as the fluid going into the borefield (i.e. the inlet temperature) and is 0°C for Belgium and -3°C for The Netherlands.
In Switzerland, the minimum fluid temperature is defined as -1.5°C average fluid temperature. Therefore, it is important when you are designing borefields to check which temperature the legislation is based on.
Average fluid temperature
The average fluid temperature is the most straightforward temperature to work with, due to its direct coupling to the borehole wall temperature via the borehole resistance. The advantage of this temperature is that it abstracts away part of the effect of the flow rate. For example, whether the flow regime is 3 °C/0 °C or 5 °C/−2 °C, the average fluid temperature is always 0 °C, making it easier to interpret the results. This, however, means that if you want to control the absolute minimum and maximum fluid temperatures, the average fluid temperature is not directly suited.
Inlet fluid temperature
The inlet fluid temperature is the temperature entering the borefield and could be described as the worst case fluid temperature, since it is always the coldest during extraction and the warmest during injection. This is because the fluid entering the borefield is also the fluid leaving the heat pump. When your heat pump is heating the building, it extracts energy from the primary fluid, meaning that the fluid at the heat pump outlet is the coldest fluid in the entire circuit (and vice versa for cooling/injection).
If you want to put strict limits on the fluid temperatures in your borefield, the boundaries on the inlet fluid temperature will ensure that you have essentially covered all potential temperatures, ensuring that there is no violation of this limit at anytime or anywhere.
Outlet fluid temperature
The outlet fluid temperature is the temperature leaving the borefield and is the best case fluid temperature. During extraction, a cold fluid is injected into the borefield and is heated by the ground, resulting in a higher temperature at the borefield outlet. Similarly, during injection, a warm fluid is injected into the borefield, where it cools down, leading to a lower fluid temperature at the outlet.
This outlet fluid temperature can be useful when selecting the correct heat pump, since the power that can be delivered by the heat pump depends on the temperature entering its evaporator (or condenser, in the case of active cooling). If you are for example simulating your borefield with the outlet temperature and you have a minimum (outlet) fluid temperature of 2°C, you need to make sure that you heat pump can deliver the required power with 2°C going into its evaporator.
Example in GHEtool Cloud
In order to understand the effect of the type of fluid temperature on the design of your borefield, let’s consider the following example in GHEtool Cloud.
Under the ‘General’ tab in the simulation settings, you can select which of the three fluid temperatures you want to design with. If you select ‘inlet’, all the respective fluid temperatures will be redefined as inlet fluid temperatures, and similarly with ‘average’ and ‘outlet’ temperatures.
In the temperature profile below, a simulation is carried out with 4 boreholes of 100 m and a variable flow rate with a constant temperature difference of 3 °C between the borehole inlet and outlet. It can be seen that, with a minimum average fluid temperature of 0.46 °C, the fluid stays nicely within the limits. However, as mentioned before, this does not mean that the absolute minimum fluid temperature does not cross the 0 °C threshold. Therefore, a simulation using the inlet fluid temperature is carried out.
When the same simulation is carried out using the inlet fluid temperatures, the fluid temperature now drops to −1.04 °C. As mentioned before, the inlet temperatures are always the lowest in the system, so even if the average fluid temperature is positive, the inlet temperature can still be negative. If you want your absolute minimum (and vice versa maximum) temperature to stay within certain limits, please work with the inlet fluid temperatures.
Finally, the outlet fluid temperatures are shown below. They only drop to 1.96 °C and are therefore the most optimistic temperatures.
It might be clear that the type of fluid used in the simulation will of course have an effect on the total required borehole length. This will be discussed in the following parts.
Conclusion
In this article, the three different fluid temperatures (average, inlet, and outlet) are discussed. The average fluid temperature is typically used in borefield design, since it is directly linked to the borehole wall temperature via the effective borehole thermal resistance. However, it does not guarantee that the absolute minimum and maximum fluid temperatures are within the limits. To guarantee this, the inlet fluid temperatures should be used. The outlet fluid temperatures can be used to guarantee that the heat pump can deliver its rated power.
Based on these insights, the following chapters will discuss the calculation of the required borefield size.
References
- Legislation in Brussels: Checklist (NL)
- Legislation in The Netherlands: BRL11000 Protocol 11001 (page 61-62)
- Legislation in Switzerland: SIA 384/6 §3.1.1.2