Update cite.md

Author: wouterpeere

docs/sources/cite.md

@@ -9,13 +9,18 @@ Whenever you work with GHEtool (in general), please reference this tool using th
 Whenever you use a specific functionality within GHEtool, one can also reference a specific paper/article related to this functionality.
 
 ## Hybrid sizing methodology (L2) | borefield quadrants
-Whenever you use the hybrid sizing methodology (L2 in GHEtool) or make use of borefield quadrants, please use the citations below.
+Whenever you use the hybrid sizing methodology (L2 in GHEtool) or make use of borefield quadrants, please use the citation below.
 
 > Peere, W., Picard, D., Cupeiro Figueroa, I., Boydens, W., and Helsen, L. (2021). Validated combined first and last year borefield sizing methodology. In _Proceedings of International Building Simulation Conference 2021_. Brugge (Belgium), 1-3 September 2021. https://doi.org/10.26868/25222708.2021.30180
 
 > Peere, W. (2020). Methode voor economische optimalisatie van geothermische verwarmings- en koelsystemen. Master thesis, Department of Mechanical Engineering, KU Leuven, Belgium.
 
 ## Active-passive cooling example
-Whenever you use the example of active-passive cooling, please use the citations below.
+Whenever you use the example of active-passive cooling, please use the citation below.
 
 > Coninx, M., De Nies, J., Hermans, L., Peere, W., Boydens, W., Helsen, L. (2024). Cost-efficient cooling of buildings by means of geothermal borefields with active and passive cooling. _Applied Energy_, 355, Art. No. 122261, https://doi.org/10.1016/j.apenergy.2023.122261.
+
+## Hybrid systems
+Whenever you use the methods for the hybrid systems, please use the citation below.
+
+> Peere, W. (2025). Three ways to design a hybrid geothermal heating and cooling system for an office building. In _Proceedings of Geo-Rin Conference_. Benasque (Spain), 2-6 June 2025.