C&I BESS System Efficiency & Auxiliary Power Consumption

C&I Energy Storage Cabinet — Single-Cycle Charge/Discharge Energy Link and Auxiliary Consumption Analysis Report

This report targets the 125 kW / 261 kWh C&I energy storage cabinet and analyzes the energy link of a complete DOD 0% → 95% → 0% cycle. The system's nominal AC-AC RTE is 88.5%, which — beyond PCS efficiency and battery DC round-trip efficiency — must further deduct the auxiliary consumption of the liquid-cooling unit, BMS/EMS, and dehumidifier.

• Core RTE (PCS + battery, excluding auxiliary): 88.95%
• Liquid-cooling electricity consumption per cycle at rated cooling duty cycle: approx. 5.8 – 8.1 kWh (covering the >50% duty-cycle range up to maximum power)
• Corresponding system net RTE range: 85.15% – 87.48% (from 25% cooling to rated cooling, covering typical on-site operating conditions)

1. System Parameters

2. Wiring and Terminals

This section does not involve external wiring; the focus is on energy metering points, with the following suggestions:

• Grid-side metering: bidirectional meter at the output terminal of the cabinet's grid-connection switch, used as the statistical source for $E_{\text{grid\_in}}$ / $E_{\text{grid\_out}}$.
• PCS AC-side metering: bidirectional meter at the PCS AC output port, used to verify PCS internal efficiency.
• Battery DC side: BMS DC-bus sampling, used to calculate battery DC round-trip efficiency.

3. Liquid-Cooling Unit Specifications

The unit adopts an inverter compressor + inverter circulation water pump. The master control signal is individual cell temperature ($T_{\max}$ / $T_{\min}$); the setpoint signal is outlet temperature ($T_{\text{outlet}}$, configurable via BMS).

3.1 Operating Mode Control Logic (BMS active)

Note: $T_{\max}$ / $T_{\min}$ are the maximum/minimum individual cell temperatures inside the DC cabinet; $\Delta T$ is the temperature difference between the inlet and outlet coolant.

3.2 BMS Power-Loss Protection Logic (BMS inactive)

When power is lost, the cooling/heating logic above is retained, with two additional forced self-circulation protections added:

3.3 Mode Priority

Cooling > Heating > Self-circulating; self-circulating can be directly overridden by the forced conditions in 3.2.

4. Communication

This report does not cover external communication protocols, but recommends unifying the following interfaces and data points during on-site data acquisition to facilitate RTE calculation at the upper-level EMS:

• BMS → EMS: battery DC voltage/current, SOC, SOH, individual cell temperature.
• PCS → EMS: AC-side voltage/current/power, charge/discharge status, fault word.
• Liquid-cooling unit → EMS: outlet temperature, compressor/water pump on/off status, current mode (cooling/heating/self-circulating).

5. Operating Procedure and Calculation Basis

5.1 Baseline Energy Link

• Battery DC round-trip efficiency 94%, symmetrically split into 96.95% × 96.95%
• PCS charging/discharging efficiency both 97.3%
• DoD = 95%

$$E_{\text{usable}} = 261 \times 95\% = 247.95 \text{ kWh}$$

$$E_{\text{grid\_in\_basic}} = \frac{E_{\text{usable}}}{\eta_{\text{DC-AC}} \times \eta_{\text{AC-AC}}} = \frac{247.95}{0.973 \times 0.9695} = 262.84 \text{ kWh}$$

$$E_{\text{AC\_gross}} = E_{\text{usable}} \times \eta_{\text{AC-AC}} \times \eta_{\text{DC-AC}} = 247.95 \times 0.9695 \times 0.973 = 233.91 \text{ kWh}$$

Theoretical system efficiency (PCS + battery core, excluding liquid cooling and fixed auxiliary consumption):

$$\eta_{\text{core}} = \eta_{\text{DC-AC}} \times \eta_{\text{bat\_RT}} \times \eta_{\text{AC-AC}} = 0.973 \times 0.94 \times 0.973 = 88.95\%$$

5.2 Single-Cycle Energy Link

• Charging path: grid → PCS (×97.3%) → battery DC (×96.95%) → stored 247.95 kWh.
• Discharging path: battery DC (×96.95%) → PCS (×97.3%) → AC gross output 233.91 kWh.
• Liquid-cooling and auxiliary consumption are presented as a floating branch: auxiliary consumption is added on the charging side, and deducted from PCS gross output on the discharging side.

5.3 General Formula

Let cooling duty cycle be $\delta$ (self-circulating duty cycle $1-\delta$); for a single full-power cycle the charging time $T_{\text{ch}} = E_{\text{grid\_in\_basic}} / P_{\text{rated}} = 2.103 \text{ h}$, and the discharging time $T_{\text{dis}} = E_{\text{AC\_gross}} / P_{\text{rated}} = 1.871 \text{ h}$.

$$P_{\text{liq\_avg}} = P_{\text{liq,idle}} + k_{\text{cool}} \cdot \delta = 0.25 + 2.25\delta \quad \text{(kW)}$$

$$E_{\text{grid\_in}} = E_{\text{grid\_in\_basic}} + T_{\text{ch}} \cdot \left( P_{\text{liq\_avg}} + P_{\text{aux\_fixed}} \right) = 262.84 + 2.103 \cdot \left( P_{\text{liq\_avg}} + 0.26 \right) \quad \text{(kWh)}$$

$$E_{\text{grid\_out}} = E_{\text{AC\_gross}} - T_{\text{dis}} \cdot \left( P_{\text{liq\_avg}} + P_{\text{aux\_fixed}} \right) = 233.91 - 1.871 \cdot \left( P_{\text{liq\_avg}} + 0.26 \right) \quad \text{(kWh)}$$

$$\eta_{\text{RTE\_net}} = \frac{E_{\text{grid\_out}}}{E_{\text{grid\_in}}}$$

Closed form after substitution (eliminating the intermediate variable $P_{\text{liq\_avg}}$):

$$E_{\text{grid\_in}}(\delta) = 262.84 + 2.103 \cdot (0.25 + 2.25\delta + 0.26) = 263.91 + 4.732\,\delta \quad \text{(kWh)}$$

$$E_{\text{grid\_out}}(\delta) = 233.91 - 1.871 \cdot (0.25 + 2.25\delta + 0.26) = 232.96 - 4.210\,\delta \quad \text{(kWh)}$$

$$\eta_{\text{RTE\_net}}(\delta) = \frac{232.96 - 4.210\,\delta}{263.91 + 4.732\,\delta}$$

5.4 Auxiliary Consumption Scenarios

Fixed auxiliary consumption: BMS + EMS = 200 W, dehumidifier = 60 W (continuously running), totaling $P_{\text{aux\_fixed}} = 260 \text{ W}$. The liquid-cooling unit is modeled under 8 typical states:

[1] Self-circulating trigger conditions: starts at inlet/outlet coolant temperature difference $\Delta T \geq 4\,°C$, stops at $\Delta T \leq 2\,°C$. When the battery is idle and the temperature difference is small, the liquid cooling is effectively in the "off" row (0 kW); during charging/discharging or temperature rise, it enters the "self-circulating" row (0.25 kW). If the BMS loses power and outlet $T_{\text{outlet}} \geq 38\,°C$ or $\leq 8\,°C$, it enters forced self-circulation for 120 s.

5.5 Conclusions and Recommendations

• Core RTE 88.95%; with 260 W fixed auxiliary, net RTE is approx. 88.6%; drops to 88.27% during liquid-cooling self-circulation; 86.70% at 50% cooling duty cycle.
• To guarantee the system-level 88.5% nominal RTE, from $\eta_{\text{RTE\_net}} \geq 0.885$, back-calculating yields:

  $$P_{\text{liq\_avg}} + P_{\text{aux\_fixed}} \leq 0.348 \text{ kW} \;\Rightarrow\; P_{\text{liq\_avg}} \leq 0.088 \text{ kW}$$

  That is, the combined average power of liquid cooling + fixed auxiliary per cycle must be ≤ 0.35 kW (of which the liquid-cooling unit itself ≤ 0.09 kW, essentially in the off or near self-circulating range).
• Footnote on self-circulation: per the control logic in Section 3.1, self-circulating starts only when $\Delta T \geq 4\,°C$ and is not always on. When the battery is idle with a small temperature difference, it is effectively "liquid cooling off" (0 kW) and the RTE can reach 88.62%; during charging/discharging with temperature rise it enters self-circulating (0.25 kW), giving an RTE of approx. 88.27%. The ≤ 0.09 kW control target above does not hold during BMS power-loss forced self-circulation (120 s boundary protection).

6. Cautions

1. The energy link in this report assumes symmetric charge/discharge efficiency. If the on-site PCS efficiency varies significantly across different SOC/power segments, it should be re-fitted piecewise.
2. The liquid-cooling unit's cooling/heating duty cycles differ significantly at different ambient temperatures; RTE should be calculated within the agreed temperature range.
3. If the dehumidifier runs on demand rather than continuously at 60 W, replace the fixed term in the formula with the actual duty cycle.
4. After battery aging, DC round-trip efficiency declines; the RTE calculation should simultaneously introduce an SOH correction factor.
5. All power/energy values are statistics for a single cycle and cannot be directly extrapolated to daily/monthly data.

Appendix A Symbol Definitions

A.1 System Parameters

A.2 Efficiency and Baseline Energy

A.3 Time and Auxiliary Power

A.4 Liquid-Cooling Unit

A.5 Energy and System-Level RTE

A.6 Symbol Naming Conventions

• Subscript in / out indicates energy flowing into/out of the grid.
• Subscript ch / dis indicates the charging/discharging process.
• Subscript aux indicates auxiliary systems; subscript liq indicates liquid cooling.
• Subscript idle / cool / heat / max indicates the liquid-cooling operating tier.
• Bare subscript max / min indicates the maximum/minimum individual cell temperature (not the power tier).
• Greek letter $\eta$ denotes efficiency, $\delta$ denotes duty cycle (note: distinguish from DoD), $\Delta$ denotes difference.