HP1.2 · Thermal storage and the charge–hold–discharge chain

Learning objectives

• Distinguish sensible, latent and other storage mechanisms from the useful-service boundary.
• Calculate charge energy and connection/window constraints without mixing MWh_e and MWh_th.
• Identify why large nominal capacity can still fail a time-limited duty.

Allow 30–45 minutes including derivations and exercises. All numerical values are teaching assumptions.

1. Stored heat is a state, not a delivery guarantee

Sensible storage uses a material temperature change, latent storage uses a phase transition and other technologies may use reversible reactions. Their useful output depends on material states, heat-transfer equipment and the required outlet condition. A nominal MWh_th rating must therefore state the charge/discharge endpoints and the temperature window that remains useful to the process. [H1]

In the simplified tool, a stated usable fraction translates nominal storage into heat that may be withdrawn over that window. Holding retention applies to a specified dwell period, and discharge efficiency translates the retained usable heat to the process meter. These are explicit screening assumptions, not universal material properties.

2. Work backward from the heat duty

A 2 MW_th process for six hours needs 12 MWh_th useful delivery. If holding retention is 95% for the declared dwell and discharge efficiency is 90%, required usable heat before holding is 12 / (0.95 × 0.90) = 14.0351 MWh_th. With a 90% usable fraction, nominal storage needs at least 15.5945 MWh_th before any separate engineering margin.

An 18 MWh_th store at a 90% usable fraction offers 16.2 MWh_th before holding, so the energy-capacity screen passes. But the discharge heat exchanger must also supply at least 2 MW_th at the required delivery condition throughout the event. Enough heat quantity does not establish adequate discharge power or outlet temperature.

3. Charging needs its own energy and power budget

Assume direct electric charging converts electricity to stored heat at 98% efficiency. To establish 14.0351 MWh_th, the target charge input is 14.3215 MWh_e. An eight-hour charging window requires an average 1.7902 MW_e. Use the smaller of charging equipment power and net site charging headroom to assess that window.

At 2 MW_e net charging power over eight hours, the available 16 MWh_e passes. At 1.5 MW_e only 12 MWh_e is available, so the recharge target fails even though the store itself is large enough. Adding more thermal capacity does not solve this electrical bottleneck. The first tool assumes charging precedes delivery in a separate, non-overlapping state.

4. Separate holding loss from a conversion efficiency

A retention fraction of 95% means 95% of the selected heat remains after the stated dwell under the specified environment and state. It is not a universal hourly loss rate and cannot be reused unchanged for an arbitrary hold duration. Similarly, a quoted round-trip efficiency may already include holding, auxiliaries or discharge loss.

In this case 14.0351 MWh_th becomes 13.3333 MWh_th after holding, then 12 MWh_th at the useful meter after discharge. The chain is a teaching reconciliation of target energy, not a dynamic temperature simulation. Temperature distribution, exchanger performance, pressure, cycling and standby needs remain specialist validation tasks.

5. A cheaper charging period is only one economic input

At a hypothetical charge price of 50 per MWh_e, the target charging energy costs about 716.07. The earlier boiler baseline at 40/MWh_fuel and 85% efficiency costs 564.71 for the same 12 MWh_th. Storage may pass the stated physical screen yet have a higher period energy bill. No conclusion about total project value follows from either energy cost alone.

A full assessment adds capital, electrical demand charges, integration, maintenance, source changes, production impact, backup and retirement, and may value flexibility only where evidence supports it. Do not count the same flexible heat twice as both protected process reserve and an unrestricted market service.

Worked answers and reasoning

Attempt the exercises before opening each answer.