The BTES 1.0 templates are divided into different types of regime temperature combinations for heating and cooling end-units. To clarify this aspect the different templates are the following.
LT heating + HT cooling
Working principle
The working principle of a geothermal system is divided into two main operating conditions, winter and summer. Of course, if there is both heating and cooling demand at the same time, both conditions are active.
Winter
In the winter, when there is heating demand, the heat pump will supply heating to the building. When the heating provided by the heat pump is insufficient, the boiler will deliver the remaining required heating. While the heat pump is delivering heat, it’s also extracting heat from the BTES system on the evaporator side. Extracting heat is the same as injecting cold, so in this operating condition the BTES system will cool down.
BTES 1.0
Heat Pump (HP)
Boiler
Low Temperature (LT) heating equivalent end-unit
Hot storage vessel
Cold storage vessel
Summer
In the summer, when there is cooling demand, the cooling stored in the BTES system (in the winter) will be used to cool down the building. If the delivered cooling from the BTES system is insufficient, the chiller will deliver the remaining required cooling. When cooling is extracted from the BTES system en supplied to the building, the BTES system will heat up, increasing its overall temperature.
BTES 1.0
Chiller
High Temperature (HT) cooling equivalent end-unit
Thermal balance soil
The soil temperature without an ATES or BTES system would be almost constant throughout the year. If an ATES or BTES system is used, and we start in January, the temperature of the soil would gradually drop in the winter months because heat is extracted from the soil (= injecting cold). In the summer months, the soil temperature will gradually rise because the cold is extracted from the soil to cool down the building. After the summer, the winter comes again, meaning the temperature will drop again. This is visualised in the figure below.
If the same amount of cold is extracted from the soil, as there is injected, the temperature at the end of the year would be exactly the same as in the beginning. This is called a thermally balanced soil.
However, if more cold is injected than there is extracted, the temperature would gradually drop every year. This is visualised in the graph below which shows the soil temperature in a 3-year simulation. In this case, the BTES is after a few years too cold.
The same thing can be said if the extracted cold is larger than the injected cold. The temperature will gradually rise every year.
A thermal imbalance of the soil reduces the system efficiency of the system because of the increasing thermal losses in the soil and the lowered SCOP value of the heat pump. It can even lead to a thermal breakdown of the system, meaning no cold can be injected or extracted anymore.
For this reason and others, more options are available in the template to give the user some strategies to make sure the thermal balance is met with an optimal system efficiency. The system with all options is visualised in the figure below.
BTES 1.0
Heat Pump (HP)
Boiler
Chiller
Low Temperature (LT) heating equivalent end-unit
High Temperature (HT) cooling equivalent end-unit
Hot storage vessel
Cold storage vessel
Dry Cooler (DC) used in different ways
To optimise the system, the user should first check if the BTES system cant deliver more cooling to the system than it already does, and check if the heat pump cant deliver more heating. So the first step is to optimise the heating and cooling contribution of the heat pump and BTES system. More information on hydraulic configurations can be found in Hybrid production Heating.
If for instance there is a lot of simultaneous heating and cooling, it might be more optimal to bypass the BTES system. In this case, the heating and cooling produced by the heat pump will both be used directly instead of using indirect cooling by first storing the cold produced by the heat pump inside the BTES system. Because less energy is stored in the BTES, the thermal losses decrease as well.
After the system is optimised by increasing the BTES and/or the heat pump contribution, the thermal balance of the BTES system should be optimised. To clarify all the different options, the BTES system is deemed either too cold or too hot.
BTES too cold
If the BTES system is too cold the following things can be done:
If there is no dry cooler or any regeneration unit available, the heat pump can be limited in storing cooling inside the BTES system. This option is suboptimal because the heat pump is a preferable production unit. If the heat pump is limited, it won’t deliver anymore heating to the building, resulting in a lower heating contribution of the heat pump and a higher contribution of the boiler.
A dry cooler or any other regeneration unit can be used to release the excess cooling to the ambient air instead of storing it in the BTES system. In this case, the heat pump sort of works like an Air Source Heat Pump (ASHP) instead of a Ground Source Heat Pump (GSHP).
A dry cooler is used to release the excess heat stored in the BTES system when there is no cooling demand.
BTES too hot
If the BTES system is too hot the following things can be done:
If there is no dry cooler or any regeneration unit available, the BTES system can be limited in supplying cooling to the building. This option is suboptimal because the cold used from the BTES system is prefered over the produced cooling by the chiller. If the BTES system is limited, it won’t deliver anymore cooling to the building, resulting in a lower cooling contribution of the BTES system and a higher contribution of the chiller.
A dry cooler or any other regeneration unit can be used to deposit cooling from the ambient air in in the BTES system (which is the same as blowing away the excess heat inside the BTES system).
Notice that implementing a dry cooler between the cold thermal store and the BTES system increases the flexibility of the installation because it can be used to prevent a BTES which is too cold or too hot (option 2 in both cases).
Free cooling
Controls: default without all bells and whistles but with the boiler and chiller
Boiler and chiller can be removed, …
More information on BTES BC, HP, boiler, chiller, configurations
HT cooling + HT and LT heating separate
HT cooling + HT and LT heating together
LT heating + LT and HT cooling separate
LT heating + LT and HT cooling together
HT and LT heating separate + LT and HT cooling separate
HT and LT heating together + LT and HT cooling separate
HT and LT heating separate + LT and HT cooling together
HT and LT heating together + LT and HT cooling together
