Use Case 5: Holzkirchen, Germany

General

The Holzkirchen geothermal plant in Bavaria uses a Turboden Organic Rankine Cycle (ORC) to convert ~140 °C reservoir heat from ~4,800 m depth into electricity and district heat. The ORC is a closed loop with an organic working fluid and offers 3.6 MW electrical capacity, wide part-load flexibility (10–110%), and >98% availability. Use-Case V aims to smooth year-round thermal withdrawal by optimizing how available heat is partitioned among power production, storage, and district heating—maximizing well productivity while protecting system stability and reservoir sustainability.

Three temperature levels exist: high (~140 °C) currently serving the ORC and some district heating; medium (~60 °C) downstream of the ORC; and low (~14 °C) that is presently underutilized. Indicative flows are ~95 kg/s on the medium stream (typical constant geothermal flow) and an estimated ~100 kg/s for the low stream (no meter installed). Heat demand is strongly seasonal: in January the district-heating (DH) load averages ~2 MW with daily swings; by June it falls to a fraction of a megawatt and is nearly negligible in July. Today, diverting high-grade geothermal heat to DH in winter reduces thermal input to the ORC and thus electrical output—despite cold ambient conditions that would otherwise favor ORC efficiency. In summer, with low DH demand, more geothermal heat reaches the ORC, so electrical output rises.

RESTORE proposed solution (Holzkirchen)

Integration Concept

To address this exergetic mismatch, the plant will couple a large heat pump (LHP, working fluid: cyclopentane) with a thermochemical energy storage (TCES) reactor.

Operation

Charging (summer/low DH demand): the LHP lifts heat from the medium (~60 °C) and low (~14 °C) sources and delivers it to the TCES at elevated temperature, storing high-grade heat (endothermic step). Charging is prioritized when surplus/low-carbon grid electricity is available, using residual ORC heat without increasing geothermal extraction. The LHP starts via an external signal and modulates to maintain a stable charging setpoint as source temperatures/flows vary; integrated controls match output to the reactor’s demand.

Discharging (winter/high DH demand): the TCES releases heat (exothermic step) directly to the DH network at ~85 °C with an ~10 °C exchanger drop (no intermediate power conversion) so response is fast and governed by DH return temperature and flow.

Benefit

Seasonal charging/discharging shifts summer surplus into winter demand, stabilizes geothermal heat withdrawal, raises overall exergy efficiency, and lets essentially the full geothermal thermal output feed the ORC in winter to maximize electricity production when the grid is tighter.