Solar Cooling: PV vs Solar-Thermal — Is There a Better Option?

Short answer: for most applications, yes — PV-driven cooling has become the better option. The decade-long collapse in PV module prices flipped the economics: solar PV driving an electric heat pump / vapor-compression chiller now converts more sunlight into cooling, more cheaply and simply, than solar-thermal collectors driving a sorption (adsorption/absorption) chiller. Solar-thermal cooling is no longer the default — but it still wins in a specific, identifiable niche.

This article frames the choice the rest of the solar-thermal and solar-adsorption threads assume, and weighs the alternatives.

The core physics: why PV pulls ahead

Two architectures turn sunlight into cold:

	A — Solar thermal + sorption	B — PV + vapor-compression
Path	collector heats water → desorbs sorbent → sorption chiller	PV makes electricity → electric chiller/heat pump
Cooling COP	adsorption (solid, silica gel/zeolite) ~0.3-0.7; absorption (liquid, LiBr) single-effect ~0.7-0.8, double-effect 1.2-1.4 at >150 °C	vapor-compression ~3-8
Solar capture	collector ~40-65% thermal	PV ~20% electrical, but feeds a high-COP machine
Balance of system	hot + chilled loops, large heat rejection, cooling tower water, parasitics	one electrical connection, smaller heat rejection

Two thermal cycles, not one. “Sorption” on the thermal side is really two distinct cycles, and they are not interchangeable:

Adsorption — a solid sorbent (silica gel, zeolite, activated carbon) adsorbs refrigerant vapor onto its surface. Intermittent/batch, COP ~0.3-0.7, but it runs on low-grade 55-95 °C heat (even ~40-65 °C with silica gel). This is the low-temperature-waste-heat cycle.
Absorption — a liquid sorbent (LiBr/water or water/ammonia) absorbs refrigerant into the bulk liquid. Continuous, single-effect COP ~0.7-0.8, and the only route to double/triple-effect COP 1.2-1.4 — which needs a higher drive temperature (≥150 °C) and concentrating collectors.

So “double-effect” is an absorption-only capability; adsorption does not reach it. Keep them separate — most studies below are specifically one or the other (the data-center waste-heat case is adsorption; the sunbelt, Caferra, and SunBeltChiller cases are absorption).

The decisive term is the ~4-10x COP gap. Even though PV captures a smaller fraction of incident energy than a thermal collector, multiplying ~20% PV efficiency by a vapor-compression COP of 3-8 delivers more cold per roof area than ~50% collector efficiency times a single-effect sorption COP of ~0.7-0.8 (and it beats even double-effect absorption’s ~1.3). PV cooling is also mechanically simpler (no second hydraulic loop, far smaller heat rejection, less water and parasitic power).

What the authoritative reviews conclude

IEA SHC Task 53 (2018) named the cause outright: solar-thermal cooling’s market struggles are “attributed to falling solar PV prices, which allowed cost-effective operation of vapor compression chillers powered by grid-connected solar inverters.” Best solar-cooling systems reach an electrical SPF of 20-40 (>80% AC electricity cut).
IEA SHC Task 65 (2024, the Sunbelt field program, 32 projects/18 countries) is blunter: PV-supported cooling “has become the dominant type of solar cooling system globally due to its simplicity in installation and low cost,” and “can result in lower levelized cost of cooling compared to solar thermal.” Only ~2,000 solar-thermal cooling systems exist worldwide vs millions of PV-driven units in Australia alone.
A 2025 head-to-head (Caferra et al.) of a Fresnel-driven single-effect absorption chiller (COP ~0.75) vs a PV compression chiller (COP ~3), both ~500 kW: the ~4x COP gap makes PV-compression win for most demand profiles; absorption is competitive only with existing/waste heat or steady high load, and needs a gas boiler backup that hurts its economics.

Economics: capex and payback decide it

Both routes have near-zero fuel cost, so upfront investment dominates the levelized cost of cooling (LCOC):

Solar-cooling capex is still 1.5-2.5x conventional AC (Task 53); a Hefei study put PV cooling capex roughly 2-2.5x cheaper than single/double-effect solar absorption (and conventional vapor-compression cheapest of all on capex).
Older thermo-economic work found payback ~6-7 yr for PV vs >20 yr for thermal at near-equal primary-energy savings.
But solar-thermal can be cheap in the right place: a 2025 Scientific Reports study of an evacuated-tube absorption chiller in Madinah, Saudi Arabia reported LCOC $0.028/kWh, ~3.0-yr payback, IRR 32% — high irradiance + high ambient + large steady load is where thermal economics shine.

Where solar-thermal cooling still wins

The thermal route remains the best option under a clear set of conditions — the decision is “free heat + steady load + cheap storage,” not raw efficiency:

Free or high-grade waste/process heat already on site. A silica-gel adsorption chiller on a data-center liquid-cooling loop paid back in ~285 days (Gupta & Puri 2021) — unbeatable when the heat is free; raising rack inlet 40→65 °C improved efficiency 16%.
Trigeneration / CHP where engine or industrial exhaust would otherwise be dumped.
Large, steady, 24/7 loads (district cooling, hospitals, industrial process cooling at high solar fraction — Task 65’s strongest thermal case).
Cheap thermal storage beats batteries, or no monetizable PV grid export exists (then thermal-with-storage delivers higher solar fraction).
Co-supplying low-temp heat (domestic hot water) year-round alongside cooling, in moderate climates (Task 53’s named niche).

The “third options” — hybrids and better collectors

The most interesting answers aren’t “A or B” but hybridize:

PV/T (photovoltaic-thermal) collectors co-generate electricity and heat from one aperture, cooling the cells (raising PV yield) with the fluid that drives a thermal cycle. Reviewed PV/T systems: absorption COP 0.615 (9.3-yr payback), adsorption effective COP up to 2.1, and PV/T + desiccant + vapor-compression at overall COP 0.68 — beating both evacuated-tube+absorption (0.34) and plain PV+vapor-compression (0.29).
Better collectors + higher-effect cycles lift thermal performance (these are absorption cycles — adsorption has no equivalent): evacuated-tube/CPC push single-effect absorption to COP ~0.75-0.81; concentrating/Fresnel collectors enable double-effect absorption (COP 1.2-1.4) and triple-effect cycles that roughly double cooling per unit heat (the SunBeltChiller hits overall COP 1.35 with storage).
Passive add-ons — radiative sky cooling and evaporative/desiccant hybrids — are flagged by 2024-2026 reviews as the frontier for closing the cost-and-COP gap.

Verdict

There is no universal winner, but the center of gravity has moved:

Default / most buildings: PV + heat pump is the better option — cheaper, simpler, faster payback, higher cooling-per-area.
Low-grade free waste heat (~40-95 °C): adsorption still wins — silica gel runs on heat too cool for absorption, so the data-center / waste-heat case this KB centers on remains valid where the heat is genuinely free.
Higher-grade heat, steady large load, or sunbelt: absorption is the thermal route — higher COP, and the only one reaching double-effect (1.2-1.4); best for district/industrial cooling and hot climates.
Best frontier bet: PV/T hybrids and better collectors/higher-effect cycles, which capture both energy streams instead of choosing.

For a project like this KB’s waste-heat-driven strategy, the lesson is precise: heat-driven cooling is justified by the free waste heat, not by beating PV on a bare rooftop. On a bare rooftop with no waste heat, PV+heat-pump is usually the better option.

Sources