How Combined Heat and Power Efficiency Depends on Useful Heat Demand

The operational reality of cogeneration is dictated by thermal absorption. To see how combined heat and power efficiency depends on useful heat demand, one must analyze what occurs when thermal consumption declines.

When heat demand drops below the prime mover’s thermal output, the facility faces two operational choices. The system can enter thermal-tracking operation (modulating electrical output down to match heating needs) or run at full electrical output while activating parasitic heat rejection.

Parasitic heat rejection uses dump radiators, evaporative fluid coolers, or cooling fans to vent excess heat into the atmosphere. This process consumes auxiliary electrical power to run fans and circulating pumps, simultaneously wasting fuel and lowering net electrical generation.

100% ┌──────────────────────────────────────────────┐ │ TOTAL POTENTIAL │ │ OVERALL EFFICIENCY │ 75% ├──────────────────────┬───────────────────────┤ │ │ Parasitic Rejection │ │ REALIZED │ (Wasted Heat & │ EFF. │ EFFICIENCY │ Cooling Energy) │ 50% ├──────────────────────┴───────────────────────┤ │ │ │ Baseline Electrical Output │ 25% │ (Engine / Turbine Shaft Power) │ │ │ 0% └──────────────────────────────────────────────┘ 0% 50% 100% USEFUL HEAT DEMAND

As useful heat demand drops to zero, the cogeneration plant functionally converts into a simple on-site generator. Because small-scale reciprocating engines and microturbines rarely match the electrical efficiency of utility-scale combined-cycle natural gas turbines, running a CHP plant without useful heat demand delivers worse fuel economy than buying grid power.

The Power-to-Heat Ratio and Prime Mover Thermodynamics

The physical delivery of thermal and electrical energy is determined by the power-to-heat ratio ($P/H$). This ratio expresses the electrical capacity divided by the recoverable heat output for a given technology.

$$\text{P/H Ratio} = \frac{\text{Electrical Capacity } (kW_e)}{\text{Recoverable Heat Output } (kW_{th})}$$

Different prime movers exhibit vastly different thermodynamic balances, which affects how sensitive they are to fluctuations in heat demand.

Prime Mover Technology	Typical Electrical Efficiency (% LHV)	Typical Power-to-Heat Ratio	Heat Delivery Medium	Sensitivity to Heat Swings
Reciprocating Engine	35% – 44%	0.7 – 1.1	Hot water (80–95°C), low-pressure steam	Moderate
Gas Turbine	25% – 35%	0.4 – 0.7	High-pressure steam (150–250°C), exhaust direct	High
Back-Pressure Steam Turbine	10% – 20%	0.15 – 0.3	Process steam, medium-temperature hot water	Extremely High
Solid Oxide Fuel Cell (SOFC)	45% – 60%	1.5 – 2.5	High-temperature exhaust, hot water	Low to Moderate

In a back-pressure steam turbine cycle, steam expands through the turbine stages to produce electricity before discharging at the exact pressure needed by industrial process lines. If process heat demand ceases, steam flow must be curtailed or routed through pressure-reduction bypass valves, collapsing power generation and system efficiency.

For stationary fuel cells, exploring how fuel cell efficiency across various loads dictates thermal output reveals that high electrical conversion leaves less recoverable waste heat, making load-matching strategies uniquely critical.

First-Law vs. Second-Law Exergetic Efficiency

A fundamental limitation of standard energy metrics is treating heat and electricity as thermodynamically equivalent. The First Law of Thermodynamics simply tallies energy quantities ($1 \text{ kWh of heat} = 1 \text{ kWh of electricity}$).

However, electricity is pure work (zero entropy), whereas thermal energy carries lower physical utility depending on its delivery temperature. This is where second-law exergetic efficiency becomes vital:

$$\eta_{exergy} = \frac{E + Q_u \cdot \left(1 – \frac{T_0}{T_{heat}}\right)}{Ex_{fuel}}$$

Where $T_0$ is the ambient sink temperature and $T_{heat}$ is the supply temperature of the recovered thermal stream in Kelvin.

When a facility generates high-grade steam from an engine exhaust but uses it merely to heat ambient space air to 20°C, the system destroys high-value thermodynamic exergy. Matching the supply temperature of the CHP system to the specific temperature profile of the heat load ensures optimal second-law performance.

High Exergy Input (Fuel) ──► Prime Mover ──┬──► Work/Electricity (High Exergy) │ └──► Recovered Heat │ ┌───────────────────────────────────────────────┴───────────────────────────────────────────────┐ ▼ ▼ High-Temp Steam Process (Industrial) Low-Temp Space Heating (Radiators) Second-Law Exergetic Efficiency: HIGH Second-Law Exergetic Efficiency: LOW

Thermal-Tracking vs. Electric-Tracking Controls

How combined heat and power efficiency depends on useful heat demand is clearest when examining control architecture. Facilities configure automated controls to follow one of two core strategies:

1. Thermal-Tracking Operation (Heat-Led)

The prime mover throttles output up or down based on the facility’s instantaneous thermal requirements.

Advantage: Guarantees that virtually 100% of recovered heat is put to productive use, keeping the fuel utilization factor near peak levels.
Disadvantage: Power generation fluctuates independently of electric tariffs. During mild weather, electricity production falls, forcing the facility to purchase peak-rate power from the grid.

2. Electric-Tracking Operation (Power-Led)

The system modulates fuel input to match the building’s electrical load, or runs at continuous base-load to maximize grid displacement and support peak shaving to lower demand charges.

Advantage: Maximizes electricity cost offsets and utility demand reduction.
Disadvantage: In periods of low thermal demand, excess heat is generated. Without a large thermal sink, cooling loops dump heat to the atmosphere, degrading overall efficiency.

THERMAL-TRACKING (HEAT-LED) Heat Demand: ▲ High ──► CHP modulates UP ──► Maximum overall efficiency Heat Demand: ▼ Low ──► CHP modulates DOWN ──► High efficiency, lower kW output

ELECTRIC-TRACKING (POWER-LED) Elec Demand: ▲ High ──► CHP runs at 100% ──► Excess heat rejected if demand low Overall Fuel Utilization drops from ~80% to ~40% during heat rejection events

The Pitfall of Part-Load Thermal Efficiency Derating

When a thermal-tracking unit throttles down, it introduces part-load thermal efficiency derating. Internal combustion engines operating below 50% load suffer lower shaft efficiency and reduced exhaust gas temperatures. This drop in exhaust temperature limits downstream heat exchanger effectiveness, reducing overall system performance.

Practical Worked Example: The Financial and Thermodynamic Impact of Heat Dumps

To see the direct impact of useful heat demand on system performance, review this practical calculation for a 500 kW reciprocating engine cogeneration system.

System Assumptions:

Fuel Input (Natural Gas LHV): $1{,}350 \text{ kW}$
Electrical Output: $500 \text{ kW}_e$ (Electrical Efficiency: $37.0\%$)
Maximum Recoverable Heat: $580 \text{ kW}_{th}$ (Thermal Efficiency: $43.0\%$)
Rated Overall Efficiency: $80.0\%$ ($1{,}080 \text{ kW} / 1{,}350 \text{ kW}$)
Fuel Cost: $\$0.05 \text{ per kWh}$ ($1{,}350 \text{ kW} \times \$0.05 = \$67.50/\text{hr}$)
Grid Electricity Value: $\$0.16 \text{ per kWh}$ ($500 \text{ kW} \times \$0.16 = \$80.00/\text{hr}$)
Displaced Boiler Fuel Value: $\$0.06 \text{ per kWh useful thermal}$ (assuming $85\%$ displaced boiler efficiency)

Operating Scenario	Useful Heat Demand (kW)	Heat Dumped (kW)	Fuel Utilization Factor (%)	Net Hourly Value Generated ($)
Scenario A: Full Thermal Sink	580 kW	0 kW	80.0%	+$47.30 / hr
Scenario B: 50% Thermal Sink	290 kW	290 kW	58.5%	+$29.90 / hr
Scenario C: Zero Thermal Sink (Full Dump)	0 kW	580 kW	37.0%	+$12.50 / hr

Breakdown of Calculations:

Scenario A (100% Demand):

Electric value = $\$80.00$. Displaced boiler heat value = $580 \text{ kW} \times \$0.06 = \$34.80$. Total revenue = $\$114.80$. Minus fuel cost ($\$67.50$) = $\$47.30/\text{hr}$ net savings.

Scenario C (0% Demand):

Electric value = $\$80.00$. Thermal value = $\$0.00$. Fuel cost = $\$67.50$. Additional radiator fan parasitics ($\sim 10 \text{ kW} \times \$0.16 = \$1.60$). Minus fuel and auxiliary costs = $\$10.90/\text{hr}$ net savings.

When useful heat demand is absent, economic margins narrow dramatically. Once maintenance contracts (typically $\$0.015 – \$0.025/\text{kWh}$) are factored in, Scenario C operates at an ongoing financial loss.

Mitigating Low Heat Demand via Decoupling and Trigeneration

Engineers employ specific technical solutions to decouple thermal consumption from simultaneous electrical demand, maintaining high fuel utilization year-round.
┌─────────────────────────┐ │ CHP Prime Mover │ └────────────┬────────────┘ │ Heat ▼ ┌──────────────────────────┴──────────────────────────┐ ▼ ▼ [ Thermal Energy Storage ] [ Absorption Chiller ] (Decouples heat from (Converts hot water into simultaneous power) chilled water for cooling) │ │ ▼ ▼ Space Heating / Hot Water Cooling / HVAC Loads

Thermal Energy Storage Decoupling

Integrating thermal energy storage decoupling (such as stratified hot water buffer vessels or phase-change storage systems) allows the prime mover to run at peak capacity regardless of immediate heating requirements.
During hours of high electrical demand and low heat consumption, heat exchangers divert thermal energy into insulated buffer tanks. When thermal demand peaks later (for example, early morning preheating cycles), stored heat is discharged without needing auxiliary boiler firing.

Absorption Chillers (Trigeneration / CCHP)

In commercial and institutional facilities, heating loads vanish during summer months while air conditioning loads peak.

By routing recovered hot water or exhaust steam into single- or double-effect absorption chillers, waste heat drives cooling cycles via lithium bromide-water pairs. This maintains steady useful thermal demand throughout summer, preserving the fuel utilization factor across all four seasons.

Pro Tip: When sizing a cogeneration unit, always size the base engine capacity to meet the minimum continuous summer thermal load (or baseload domestic hot water), rather than peak winter thermal demand. It is far more cost-effective to supplement winter peaks with a conventional boiler than to reject excess engine heat all summer.

Primary Energy Savings and Regulatory Compliance

In energy policy frameworks (including the European Energy Efficiency Directive and various US state Clean Peak Standards), cogeneration facilities must meet strict thresholds to qualify as “High-Efficiency CHP.”

The standardized primary energy savings calculation ($PES$) defines this requirement:

$$PES = 1 – \frac{1}{\frac{\eta_{thermal}}{\text{Ref } \eta_{thermal}} + \frac{\eta_{electric}}{\text{Ref } \eta_{electric}}}$$

Where $\text{Ref } \eta$ values represent separate generation reference baselines (such as 85–90% for modern condensing boilers and 40–55% for modern combined-cycle power grids).

If the useful thermal output factor drops because of inadequate on-site heat demand, the calculated $PES$ turns negative. A negative primary energy savings value indicates that the cogeneration plant uses more primary fossil fuel than generating electricity via the central grid and heating the building with a local condensing boiler. Under many regulatory programs, failing to meet positive $PES$ targets can forfeit environmental credits, capacity payments, or grid interconnection exemptions.

Practical Steps for Assessing Your Facility’s Heat Load Profile

Before selecting or refurbishing a cogeneration system, conduct an audit of your thermal demand characteristics:

Log Thermal Loads Hourly: Monthly utility gas bills hide seasonal and diurnal fluctuations. Deploy ultrasonic heat meters or fuel loggers to chart hourly heat demand over 12 consecutive months.
Identify Coincident Peak Demands: Evaluate how often peak electricity use aligns with peak thermal needs. Significant misalignments indicate a need for thermal storage or thermal-tracking controls.
Assess Return Water Temperatures: Hydronic heating circuits must return water at low enough temperatures (typically below 65°C) to condense exhaust streams and extract low-grade engine jacket heat. High return temperatures prevent effective heat capture regardless of gross demand.
Evaluate Trigeneration Potential: If significant chilled water or refrigeration loads exist during periods of warm weather, quantify whether absorption cooling can supply the missing baseload thermal sink.

FAQs About Cogeneration and Useful Heat Demand

What happens if a CHP plant generates more heat than the facility can use?

If the site cannot absorb the recovered heat, the system must either ramp down power output or direct excess hot water to cooling radiators that reject it into the outside air. Rejecting heat consumes auxiliary fan power, wastes fuel, and lowers the plant’s overall fuel utilization factor.

Can a combined heat and power system run without any heat demand?

Yes, most systems can operate mechanically without useful heat demand if equipped with external cooling radiators. However, doing so destroys the plant’s economic rationale. The facility burns expensive on-site fuel solely for electrical production at a lower efficiency than central utility stations, incurring wear and maintenance fees without offset heat savings.

How does thermal tracking differ from electric tracking?

Thermal tracking modulates the generator’s output to match the facility’s instantaneous heat needs, maximizing overall fuel efficiency while letting electricity output float. Electric tracking modulates the system to follow facility power demand, which can lead to rejected heat and degraded overall efficiency during periods of low heating demand.

What is a good power-to-heat ratio for a commercial building?

Commercial buildings with continuous domestic hot water and space heating typically perform best with a power-to-heat ratio between 0.6 and 1.0, typical of reciprocating internal combustion engines. Facilities with high continuous steam requirements, such as paper mills or hospitals, often suit gas turbines with ratios between 0.4 and 0.7.

How does hot water storage improve CHP efficiency?

Thermal storage decouples thermal production from immediate thermal consumption. It absorbs excess heat during periods of low heat demand, storing it in insulated water vessels so the prime mover can continue generating valuable electricity without dumping heat. The stored thermal energy is then discharged during heating peaks.

Final Thoughts on Useful Heat Demand in Cogeneration

The technical viability and economic payback of a cogeneration project depend on the presence of a continuous, high-volume thermal load. As demonstrated, understanding how combined heat and power efficiency depends on useful heat demand protects operators from investing in systems that turn into inefficient on-site generators.

To maximize your investment, analyze year-round thermal load durations, consider thermal buffer vessels to decouple simultaneous loads, and ensure the prime mover is sized to meet baseline heating demand rather than intermittent peaks.

How Combined Heat and Power Efficiency Depends on Useful Heat Demand

Defining Combined Heat and Power Efficiency Metrics