11.4 Biomass, Geothermal, Hydropower, Solar Thermal, and Waste-to-Energy Systems

Biomass, Geothermal, Hydropower, Solar Thermal, and Waste-to-Energy Systems

Section 11.1 covered solar PV and wind, and Section 11.2 covered CHP. The CEM Body of Knowledge's Distributed Generation & Renewable Energy domain also lists Biomass, Geothermal, Hydropower, Solar Thermal Systems, Waste to Energy, and District Energy Systems. These resources fill niches that PV/wind cannot—high-temperature process heat, firm baseload, or heat-recovery-from-waste—and the CEM must know when each is the right tool.

Biomass Systems

Biomass converts organic feedstock—wood chips, pellets, agricultural residue, biogas from anaerobic digestion, and liquid biofuels—into heat, power, or both. Conversion routes:

• Direct combustion in boilers or stoves for heat, often paired with steam turbines for CHP.
• Gasification converts solid biomass to a combustible syngas for engines, turbines, or boilers.
• Anaerobic digestion of organic waste produces biogas (≈60% methane) for boilers, CHP, or upgrading to RNG.

Carbon neutrality is the key nuance: combustion releases CO₂ immediately, but the feedstock regrew by absorbing CO₂, so the cycle is near-neutral if the feedstock is sustainably harvested and regrown. The CEM evaluates biomass on delivered-energy cost, feedstock supply security, ash handling, and emissions, not just on theoretical neutrality.

Geothermal Energy

Geothermal serves two distinct markets:

• Ground-Source Heat Pumps (GSHPs / geoexchange) use the shallow ground's near-constant temperature (≈50–60°F) as a heat source in winter and a heat sink in summer. Because the loop exchanges with a mild source rather than cold winter air, GSHPs achieve COPs of 3–5, well above air-source heat pumps. Loop types include closed-loop vertical, horizontal, and pond, plus open-loop groundwater.
• Deep geothermal taps hydrothermal reservoirs for direct steam or hot water to drive turbines or supply district heat—economical only where the resource is close to the surface (e.g., the U.S. West, Iceland).

GSHP economics hinge on the loop installation cost (often the majority of first cost) and ground thermal conductivity; the CEM models lifetime COP against the local electricity and fossil-fuel prices.

Hydropower

Hydropower converts the energy of falling water: P = η · ρ · g · Q · h, where η is turbine/generator efficiency, ρ water density (1000 kg/m³), g gravity (9.81 m/s²), Q flow (m³/s), and h head (m).

For facilities with on-site streams, micro-hydro (typically <100 kW) and run-of-river schemes (no large reservoir) can supply firm baseload at very low operating cost. The catch is capacity factor: stream flow varies seasonally, so the CEM sizes the turbine to the firm (exceedance) flow, not the peak, to avoid chronic under-generation. Hydropower is among the highest-capacity-factor renewables when the resource is stable.

Solar Thermal Systems

Unlike PV (which makes electricity), solar thermal makes heat. Collector types:

Applications include solar hot water (the most common), solar space heating, and Solar Industrial Process Heat (SHIP) for food, textile, and chemical operations. The CEM sizes collectors against the load's temperature and the local solar irradiance (kWh/m²·day), and accounts for freeze protection, storage, and backup. Evacuated tubes outperform flat-plate at high delivery temperatures or in cold climates because the vacuum cuts convective loss.

Waste-to-Energy (WtE)

Waste-to-Energy recovers heat from municipal solid waste (MSW):

• Mass-burn incinerates MSW as-received; the heat raises steam for power or district heat.
• Refuse-Derived Fuel (RDF) processes waste into a more uniform fuel before combustion.

WtE serves a dual purpose—volume reduction of landfill-bound waste and energy recovery. The CEM evaluates WtE on tipping-fee revenue, emissions control (acid gases, dioxins, mercury), ash disposal, and base-load dispatchability (WtE runs ~24/7, unlike wind/solar). It pairs naturally with district energy.

District Energy Systems

District energy centralizes heating and/or cooling production and distributes it through insulated pipe networks to many connected buildings. A central plant can use CHP (Section 11.2), biomass, WtE, or large chillers/heat pumps at efficiencies and economies of scale no single building can match. Benefits:

• Higher efficiency from large, well-maintained central equipment.
• Fuel flexibility—switch the central plant without touching customer buildings.
• Space recovery in connected buildings (no local boiler/chiller rooms).
• Resilience—redundant central plants ride through outages.

The CEM evaluates district connection against the connection fee, per-unit energy tariff, and the avoided cost of in-plant equipment replacement. For dense campuses, hospitals, and downtown cores, district energy is often the lowest-life-cycle-cost option.

Selecting Across the Renewable Mix

The CEM matches the resource to the load's character:

• High-temperature process heat → solar thermal (concentrating) or biomass.
• Firm baseload with on-site water → micro-hydro.
• Heating/cooling with mild ground → GSHP.
• Waste stream + heat demand → WtE or biogas CHP.
• Dense multi-building load → district energy fed by any of the above.

No single renewable fits every site; the CEM's value is selecting—and often combining—the right resource for the load, climate, and waste stream at hand.