Esslingen's Electric Bus AC Project: What Fleet Buyers Can Learn from the HVAC Layout

The Esslingen project news

On July 17, 2026, Eberspächer described its AC135-230 rooftop system for Esslingen's 52-bus battery-trolleybus program. The release reports 30 kW of cooling and 35 kW of heating output for the system, with a separately mounted electric compressor, and says deliveries would proceed in phases through the end of 2026. Those output figures are not electrical-input figures, and the planned program total should not be read as confirmation that all buses had already entered service. [Source]

The project is a useful reference for buyers because an electric bus combines passenger comfort, restricted roof space and an energy budget that changes with the route. The HVAC package has to work within that complete vehicle arrangement. A large cooling number alone does not describe whether the proposed bus can deliver its intended service.

This article turns the announcement into a set of questions for fleet tenders and OEM projects. It does not claim that VETHY or CTNET participated in the Esslingen program. Any VETHY-led bus HVAC proposal needs its own defined scope, partner responsibilities and application evidence.

Start the tender with the route

A bus route determines much of the HVAC duty. Stop frequency, door opening, passenger loading, traffic and layovers influence the occupied environment and energy use. Two buses in the same city can have different thermal demands if their routes and schedules differ.

Provide suppliers with representative operating sequences. Include the duration of travel and stops, the expected occupied periods and the conditions at the start of service. State whether the bus begins from a conditioned depot, is preconditioned while connected to power or starts after outdoor parking.

The route description should identify the difficult cases rather than only an average day. A long layover in direct sun or a crowded repeated-stop period may require separate evaluation. The buyer should explain which cases govern acceptance and which are used for planning.

This information lets suppliers discuss capacity and control strategy in context. It also gives the fleet a basis for checking the proposal after delivery. Without a route-based brief, a tender can produce several nominally compliant systems that were evaluated against very different assumptions.

Separate passenger comfort from equipment protection

An electric bus can have cabin or passenger HVAC alongside battery and power-electronics cooling. The functions may interact, but their targets are different. The tender should identify which functions are included in the HVAC supplier's scope and which belong to another system.

Passenger requirements should state the intended occupied conditions and the locations used for evaluation. Equipment requirements should come from the responsible vehicle-system designers. A single temperature target cannot represent all functions.

If resources are shared, define the simultaneous operating cases and control priorities. The vehicle manufacturer should explain what happens when available power or capacity is constrained. The fleet needs to understand any operating limitation and how it is communicated to the driver or maintenance team.

Keep the responsibility map visible. The HVAC supplier, battery-system supplier and vehicle integrator should not each assume another party has validated the combined behavior. A clear map identifies who defines, implements and verifies every important interface.

Cooling output is not the bus's electrical draw

Thermal output and electrical input measure different things. The tender should request both at defined conditions, including the relevant auxiliary loads. A fleet cannot calculate route energy by multiplying a cooling-output rating by operating hours and treating the result as battery consumption.

For an illustrative calculation, suppose a hypothetical HVAC installation averages 8 kW of electrical input during a two-hour route segment. Its electrical energy for that segment would be 16 kWh. If another segment averages a different input, calculate it separately. These values are hypothetical and are not performance data for the Esslingen system or a VETHY product.

The average depends on conditions and control behavior. Door openings, ambient temperature, solar load and passenger demand can change the operating pattern. The buyer should ask for an application estimate with assumptions, then verify the installed vehicle under the agreed duty.

The electrical budget should include the rest of the bus. HVAC is one load among propulsion, auxiliary systems and any other relevant functions. A reduction in HVAC energy does not translate directly into the same percentage increase in vehicle range. Keep subsystem and whole-vehicle calculations separate.

Evaluate off-wire or battery operation as its own state

For a trolleybus or another vehicle with multiple power modes, the thermal system may operate under different resource constraints in different states. The project should define the HVAC behavior for each state rather than assuming a single continuous supply condition.

Identify which modes are available during travel, stops and depot operations. The vehicle integrator should confirm the electrical allocation and any limits associated with each mode. If energy-management decisions can affect comfort, the fleet should understand the conditions and intended response.

The test plan should include transitions between the relevant modes. A system can perform well under a steady supply yet behave differently when the vehicle changes state. Record the control requests, available power and achieved conditions through the transition.

Do not infer a route capability from a battery or HVAC specification in isolation. The complete vehicle must meet the operating requirement. A fleet tender should request evidence for the intended route and power arrangement, with any limitations stated clearly before acceptance.

Roof space is a shared engineering resource

Electric buses may place several systems on the roof. The HVAC package must coexist with other equipment, structural requirements and maintenance access. A compact design can be valuable, but its benefit should be evaluated in the actual vehicle layout.

Ask for a roof plan showing equipment, airflow paths, connection routes and service zones. Include the space needed to remove covers or replace assemblies. A roof that appears fully utilized in a drawing may leave insufficient access for routine maintenance.

The structural and mounting review belongs to the responsible vehicle engineers. Equipment dimensions and mass should be defined on a consistent basis, and the completed installation should be assessed for the intended operating environment. A supplier's installation on one bus does not automatically approve the same arrangement on another.

The buyer should also consider production sequence. Some connections may be easy to make before adjacent equipment is installed but difficult to inspect afterward. The design should support both assembly and service, with documentation showing how each task is performed.

Air distribution matters across the passenger compartment

A bus is a long occupied space, often with changing passenger density and frequent door activity. The HVAC specification should address distribution, not only total capacity. The fleet should identify the positions and conditions used to assess passenger comfort.

The vehicle layout can create zones with different exposure or airflow. The front, rear and areas near doors may not behave identically. A measurement plan should reflect the actual passenger environment rather than relying on a single sensor near an outlet.

Ventilation requirements should be stated separately from temperature targets. If the proposed system varies fresh-air delivery, describe the control approach and the conditions under which it changes. The fleet needs to understand the service being provided, not only the energy consumed.

During sample testing, record the bus configuration and operating state. Door cycles, occupancy assumptions and fan settings should be consistent with the agreed protocol. A clear test record helps the fleet interpret the result and compare it with other proposals without turning a short demonstration into an unsupported universal claim.

Heating needs its own operating review

A bus HVAC tender should not assume that adequate cooling automatically establishes adequate heating. The heating architecture, energy source and operating limits need a separate description. Cold starts and sustained winter service can create different requirements.

Ask how the proposed system reaches the required occupied condition and maintains it through the route. Identify whether preconditioning is part of the operating plan and which energy source supports it. If the proposal uses several heating functions, define how they are coordinated.

The comparison should preserve equivalent ventilation and comfort requirements. A lower energy result obtained by providing a different service should not be presented as a direct efficiency improvement without explanation. Test boundaries and conditions need to remain visible.

The fleet should also understand service and fallback behavior. If a heating function is unavailable, what does the vehicle report and what action is required? The answer should follow the approved vehicle design and operating requirements, with clear responsibilities for the driver and maintenance organization.

A route-based acceptance scenario

Consider a hypothetical fleet evaluating a bus HVAC proposal for a repeated-stop urban route. The fleet first defines a representative sequence with a warm start, a busy boarding period, a quieter travel period and a layover. It selects measurement locations and states the intended passenger conditions.

The supplier and vehicle integrator then identify the configuration to be tested, including hardware, software, ventilation settings and available power. The test record captures ambient conditions and any departure from the planned sequence. This makes the result interpretable rather than a collection of disconnected readings.

The fleet reviews both achieved conditions and energy use. If one zone does not meet the agreed target, the team investigates air distribution, control and installation before assuming that a larger nominal capacity is the answer. If energy use differs from the estimate, the team checks the assumptions and measured operating pattern.

Any corrective action is documented and evaluated in the relevant condition. A successful retest should identify the final configuration. The acceptance record then supports production control and future service, because the fleet knows what arrangement produced the agreed result.

This scenario is an evaluation method, not a claim about an actual customer trial. It demonstrates how a route can become a repeatable technical requirement and how a supplier's proposal can be assessed through evidence relevant to fleet operations.

Service planning should begin before the tender closes

The fleet needs a repair route for the equipment it purchases. Ask the supplier to define routine maintenance, diagnostic access, replacement scope and any specialized tools or training. The proposal should identify which tasks the fleet can perform and which require outside support.

Roof-mounted equipment creates access considerations. The maintenance organization should review the proposed arrangement and determine whether its facilities and procedures are suitable. A design that meets packaging targets but complicates ordinary service can affect vehicle availability over time.

Spare-parts identification should follow the accepted configuration. Controllers, fans and other service items may vary across versions. The fleet should receive a clear parts and documentation route rather than a generic family catalog that leaves technicians to infer compatibility.

Ask how field information returns to the supplier. A structured report should capture the vehicle, operating condition, fault information and relevant observations. This helps distinguish an equipment fault from an installation or control issue and supports a more efficient technical response.

Write tender evidence requirements proportionately

The tender should request evidence that supports the actual decision. Product data, interface drawings, relevant test reports, production controls and service commitments can each answer different questions. Avoid asking for broad documents without examining their scope.

A supplier's historical project can demonstrate experience, but it does not automatically validate the proposed configuration. Ask what is carried over and what changes for the new vehicle. The response should identify which evidence remains applicable and which needs additional work.

For a cooperative supply arrangement, state the roles of the lead company, manufacturing partner and vehicle integrator. The customer should know who signs the relevant scope, who manages technical decisions and who is accountable for delivery and support. These roles should be confirmed in the actual project documents.

The evaluation should distinguish evidence already available from commitments to produce evidence later. Both can be appropriate at different stages, but they should not receive the same interpretation. A clear evidence register helps the tender team compare proposals fairly and follow through after award.

Connect manufacturing experience to the new project

An experienced manufacturing partner can contribute established processes and application knowledge. The lead project company can contribute customer coordination, requirements development, design work or other agreed functions. The value comes from how those responsibilities are connected for the new program.

For VETHY and CTNET discussions, the public cooperation information can provide context, while the customer's project should define its own scope and evidence requirements. Historical partner projects must retain their actual attribution. They should not be described as VETHY-led awards or deliveries without supporting records.

This distinction is useful to large customers. It allows them to assess the combined offer without uncertainty about who did what. A clear proposal can emphasize VETHY's intended lead responsibilities for a new project while identifying the manufacturing and validation activities assigned to the partner.

The resulting document should explain the route from customer requirement to design, sample, testing, production and service. That route is a stronger demonstration of capability than a general statement that the parties can handle every bus HVAC application.

A route-energy table with visible assumptions

A simple phase table helps a fleet keep electrical input separate from thermal output. The following values are entirely hypothetical and do not describe the Esslingen buses, Eberspächer equipment or a VETHY proposal. They illustrate how a route estimate can be organized before measured vehicle data is available.

The energy in each row is input power multiplied by time. The total is the sum of those rows. It excludes propulsion and other vehicle loads and therefore cannot establish the bus's total battery use or range. It also depends on the assumed averages, which must be replaced by relevant data before a procurement commitment relies on them.

The table makes several useful questions visible. Is initial conditioning supplied from the vehicle battery or an approved external source? Does the busy period include the intended door-opening pattern? Does the quieter period still provide the required ventilation? If the route changes, which row changes and by how much?

The fleet should add the rest of the vehicle energy model using consistent boundaries. Do not count an auxiliary load twice if it is already included in a measured whole-vehicle figure. Likewise, do not omit a load merely because another supplier assumes the vehicle integrator has included it.

When a trial is performed, record the measured energy by phase where practical. A difference from the estimate can then be investigated. The initial condition may have been more demanding, the route may have lasted longer or the equipment may have operated differently from the assumed pattern. The table provides a place to explain the result instead of applying an unexplained adjustment to the final total.

For tender comparison, ask competing suppliers to use the same route description and cabin requirements. They may propose different equipment or controls, but their estimates should make the assumptions clear. A smaller predicted energy total is not meaningful if it relies on a different ventilation service or a less demanding starting condition.

The accepted table should retain its status: preliminary estimate, supplier model or measured result. Those labels matter when the information is later reused in a fleet business case or website article. An illustrative calculation should never become a claimed vehicle test simply because it appears in a polished document.

This method helps the fleet discuss energy without losing sight of passenger service. The objective is to meet the required conditions with a justified operating budget, not to minimize one number by leaving the actual route and comfort requirements undefined.

Frequently asked questions

Is the reported cooling output the electrical input?

No. Thermal output and electrical input are different quantities. A fleet needs input data under relevant conditions to estimate energy use. The published output figure alone cannot establish battery consumption over a route.

Does a 52-bus program announcement confirm that every bus is already operating?

No. The cited release describes phased deliveries. Program quantity, delivery status and operating deployment should be reported separately, using the evidence available for each.

Can the same rooftop system be fitted to any electric bus?

No. Power, packaging, structural interfaces, controls and operating duty must be reviewed for the specific vehicle. A successful reference application is relevant experience, not automatic compatibility with another platform.

What should a fleet send VETHY for an electric bus HVAC inquiry?

Provide the bus type and drawings, route duty, climate, passenger requirements, available power, required control interfaces and quantities. State the desired project roles and acceptance process. VETHY can then discuss a defined cooperation scope and the evidence needed for the proposed configuration.

Make the route the center of the HVAC decision

The strongest bus HVAC proposal connects the equipment to the service the fleet must operate. It explains passenger conditions, energy use, packaging, control behavior and recovery in one coherent project. A capacity rating becomes useful within that explanation rather than standing in for it.

Use industry project news to ask better questions about the next vehicle, while keeping the original project's attribution and results intact. That approach creates a more credible tender and a clearer path from supplier selection to buses that can be accepted and supported in service.