Exergy - eXm Exergetic Heat Engine

Enhanced Efficiency in Heating Systems, Heat Recovery, District Heating Networks, Heat Pumps, and Domestic Hot Water Systems

Why Was the eXm Heat Engine Invented?

Circulation in Domestic Hot Water Networks

Circulation in domestic hot water networks generates varying return temperatures depending on water usage levels. In pure circulation mode, the return temperature can rise to 57-58°C. By implementing fresh water technology (FWE-Vfn fresh water system + buffer tank), high return temperatures can be distributed in the middle of the buffer tank on the heating side through switching and lower-layer channels. However, if the circulation share is so high compared to draw-off operations that a cold zone cannot develop in the lower part of the tank, an excessively high return temperature will inevitably reach the heat source at some point. This leads to unfavorable operating conditions for various heat generators, such as condensing boilers, district heating networks, combined heat and power plants, or solar systems.

Domestic Hot Water Circulation

Lost Energy

Recovered Energy

Buffer Stratification - Thermal Imaging Study

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Exergy and Anergy = Energy

The energy of a system or the energy transported by heat flow can be divided into two parts: exergy and anergy = energy.
Exergy describes the portion of energy that can be converted into work; in other words, exergy is the ability to perform work.
On the other hand, anergy is the part of energy that is useless and cannot be converted into work.
Exergy is the useful part of energy.

• This is why exergy is referred to as “useful energy.”
• In contrast, anergy is “useless energy.”

Example
To provide lighting, we use a good old light bulb (light = exergy). To do this, we need electricity (electricity = energy). Anyone who has ever touched a light bulb knows it gets extremely hot after a short time (heat = anergy). The heat becomes useless. Exergy is not just about quantity but also quality!

The law of energy conservation expresses the experience that energy is a conserved quantity, meaning the total energy of an isolated system does not change over time. It is true that energy can be transformed into different forms, such as from kinetic energy to thermal energy. Energy can also be transported to or from a system. However, it is impossible to create or destroy energy. Energy conservation is a fundamental principle in all natural sciences.
Unlike energy, exergy is not a conserved quantity. It is lost in many processes but can never increase unless supplied externally. In other words, the highest possible exergetic efficiency must be achieved at every step. This is because the loss of exergy means the loss of opportunities for further transformations or energy utilization. The key to understanding anergy and exergy is that these two state variables can only be determined by considering the environment. To make this clearer, one could ask the following question: How much would we pay for a glass of water?

Free Water                                       Priceless Water

It always depends on the environment, such as where we are when drinking a glass of water. The environment is also crucial for exergy. For heating and domestic hot water, the temperature level must be generated appropriately—not too low (low useful energy) but also not too high (poor energy efficiency).
In summary, any system that deviates from environmental conditions (pressure, temperature, concentration), whether upward or downward, contains exergy. The more it aligns with environmental conditions, the higher the anergy share becomes, until the system is energy-free only when it is in equilibrium with the environment’s state.

Let’s Use That Heat More Effectively

Is the supply temperature too low? Is the return not cold enough? Are the switching cycles of the heat generator too short?
The only way to increase the amount of heat under given conditions is to increase the temperature difference (spread) between supply and return.
The eXm heat engine increases the temperature level in the heat storage tank regardless of current consumption, enhances the effect of traditional or hybrid systems, and minimizes switching cycles.

The eXm Heat Engine Increases Temperature Spread

• More energy can be transported!
• Pipe diameters can be reduced!

Selecting the exergy level in a defined system is a challenge. Closed considerations (heat generation - heat distribution - heat exchange - heat utilization) must be taken into account.

Stratification Efficiency

Temperature stratification in water tanks is automatically set based on gravity and temperature-dependent water density. However, compensatory processes counteract this natural process, which can generally be attributed to three causes:

• Heat conduction and diffusion in water and within the built-in tank
• Induced current
• Kinetic energy of direct loads

What Destroys Stratification - Buffer Layering?

<span-первых style="font-family: helvetica;">In the case of direct loading and unloading of the tank, it is primarily kinetic energy and entraining flow that cause the mixing of fluid layers at different temperatures. Entrainment flow can be prevented by introducing the fluid into the tank at the appropriate height—i.e., at a height corresponding to its temperature. If the optimal vertical position is unknown or variable, such as during solar heat loading or return from a building’s heat distribution room, layered loading devices can be used to place the fluid at the appropriate storage height. The absence of a layered loading device means that the fluid, on its predetermined path in the storage device due to gravity and momentum, carries surrounding fluid layers and mixes with them. However, this does not necessarily mean that the existing storage layer will be destroyed.
Loading at high volumetric flow rates causes turbulence and currents in the tank, which can destroy the existing storage layer. Potential damage to storage stratification and the resulting reduction in system efficiency are usually much greater than with even imperfect stratification.

Operating Principle of the eXm Heat Engine

It draws heat at a medium level from the heat storage tank, raises the temperature, and delivers heat to the upper part of the tank. Simultaneously, eXm produces cold, thus servicing the lower part of the tank. The eXm heat engine generates both heat and cold simultaneously, making it appropriately efficient (similar to a refrigeration cycle: evaporation -> compression -> liquefaction -> relaxation).
The supply temperature increases, and the return temperature decreases. With the temperature spread ΔT, thermal energy increases proportionally (Q = c * m * ΔT).

The efficiency of the heating system can be increased with minimal energy consumption. Desired low return temperatures are maintained, high heating temperatures are sustained, waste heat is utilized, and sources such as local and district heating, solar modules, or CHP are optimally used.
The eXm heat engine is an innovative solution to efficiency challenges.

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What Are the Advantages of the eXm Heat Engine?

The COP value of eXm is not the decisive factor here. You must always keep an eye on the entire system.
eXm ...
... creates defined operating conditions—regardless of how much energy is currently supplied or drawn from the system
... raises heat to a higher, more usable temperature level (exergy)
... ensures low return temperatures
... increases operational safety
... enhances the efficiency of heat generators
... minimizes wear on heat generators through longer switching cycles

eXm Heat Engine - Technical Specifications

Comparison - District Heating with and without the eXm Heat Engine

System B with eXm and System C without eXm BIZ-BW Mannheim

For comparison, two identical systems at the BIZ-BW (Bundeswehr Education Center) in Mannheim were considered. Both system parts, B and C, are of the same size and have roughly the same purpose. System B was retrofitted with the eXm exergy heat engine.
System C is conventionally operated without eXm, meaning stratification is destroyed by consistently high return temperatures due to hot water circulation without actual hot water consumption.
Measurements show at first glance that switching cycles (identifiable by the opening and closing of the FW valve) in System B with eXm are significantly reduced over 24 hours compared to System C, specifically in System B (5 times) and in System C (17 times).
Stratification in the tank can primarily be identified by the difference (spread) between the highest temperature in the tank (SP-FWE1) and the lowest temperature in the buffer (SP-RES2). The greater the temperature difference in the tank (from top to bottom), the better the stratification in the tank. Measurement comparison shows a max. difference between the top and bottom of the storage tank Δθ_max= 50 K in System B and Δθ_max = 20 K.
In addition to lowering the return temperature, the eXm exergetic heat engine process also increases the supply temperature. The table shows a min. return temperature t_RLmin = 25°C in System B and t_RLmin = 42°C in System C. The maximum supply temperature in System B is t_VLmax = 75°C, and in System C t_VLmax = 68°C.

Direct Comparison of Systems B and C with the eXm Heat Engine - Weekend Operation

Direct Comparison of Systems B and C with the eXm Heat Engine - Weekday Operation

Conclusions from Measurements with the eXm Heat Engine

• Switching cycles for district heating top-up reduced by approximately 70%.
• Stratification increased by more than double.
• Return temperature decreased by over 70%.
• Supply temperature increased by 7 K.
• Reduction of the primary district heating return temperature by 38% to an average of 33°C.

By reducing switching cycles, less district heating is effectively needed, with the side effect that the valves and pumps used are protected by lower switching frequency.
Utilizing the middle storage layer is only possible with eXm, enabling stratification in the tank and a low return temperature for district heating. With low return temperatures, district heating is utilized much more effectively, resulting in:

• Technical operating conditions are adhered to at all times.
• Penalties for the user are avoided.
• Network efficiency is increased, while network losses for the district heating operator are reduced.

In Which Applications Does Using eXm Make Sense?

The use of the eXm heat engine is suitable wherever low return temperatures, elevated supply temperatures, reduced switching cycles, extended operating times, or high circulation losses are required.

Heat Pumps - Increase Supply Temperature, Reduce Switching Cycles, and Extend Operating Time

Many heat pumps (air/water) typically deliver a supply temperature of up to 50°C—sufficient for surface heating but not hot enough for hygienic domestic hot water preparation. The eXm device raises the temperature to 65°C or higher, making the heat suitable for high-temperature heating or domestic hot water preparation. Additional heating with electric heaters or gas boilers can be avoided. Operating and pause times are extended, meaning the heat pump is switched on much less frequently. On cold winter days, the heat pump can operate at a lower temperature level, thus more efficiently.

Energy Efficiency with the Exergetic Machine in a Passive House in Austria

A Passive House Should Be Comfortable, Smart, and Efficient...

"Quality of life must be organized to some extent," says Günter Morscher, owner of Morscher Bau- und Projektmanagement GmbH. His contribution to this is efficient homes with comfort—like the four passive houses in Unterkrumbach in Bregenzerwald. The residential complex, with over 3,200 m² of living space, was built over the past five years and provides a home for a total of 43 families.

Using Wood Saves Energy

The use of wood on the facade, shingles, and in interior fittings makes the properties blend into the environment while increasing residents’ comfort. "Wood also helps save energy," explains Morscher, "because rooms with untreated wooden floors and wall elements are perceived as cozier." This alone can lower the room temperature by about half a degree without sacrificing comfort.

"Smart Efficiency" Enhances the Passive House Concept

But even more energy can be saved on the technical side. The passive house concept still offers many possibilities, says Morscher. "The next logical step is the further development of the passive house toward smart efficiency." The eXm system from Varmeco (Kaufbeuren) provides intelligence and efficiency in new residential buildings. It optimizes building heating technology, each powered by a geothermal heat pump and solar system, by increasing the exergy share.

Usable Heat with the Lowest Possible Energy Input

Exergy is the usable part of energy, i.e., in the case of a heating system, the energy that can be used for heating and domestic hot water preparation. Even water heated to 15 degrees contains thermal energy—but it would be useless for bathing or heating. Only at a higher temperature level does this energy become exergy. The task of the eXm system supplied by Varmeco is to effectively achieve usable temperature levels. For each building, this includes a high-temperature and low-temperature heat storage tank, a module for connecting the solar system, a domestic hot water heater, as well as the so-called eXm automation (here in a 10 kW version) and the VarCon380 system controller. Above all, the eXm device and system controller provide the heating system with that extra bit of intelligence and energy efficiency.

The eXm Machine Provides the Necessary "Heat Boost"

"In the past, in such properties, we would have had to work with two heat pumps for heating to achieve the appropriate temperatures for hygienic domestic hot water preparation," says Morscher. "That was not efficient, especially in winter. Now, the eXm machine provides the necessary temperature increase with significantly lower energy consumption, raising about one-third of the thermal energy from 35 degrees to over 60 degrees." The solar yield also benefits from this "heat boost," as, in spring or autumn, if the system delivers only moderate temperatures, the eXm machine raises them to a usable level.

Side Effect: Heat Sources Operate More Efficiently

The eXm device itself is a special type of heat pump and operates only when needed, i.e., when the temperature in the warmer area of the heat storage tank falls below a specified setpoint. The eXm machine then tops up the hot area. At the same time, it cools the lower area of the low-temperature storage tank. This not only efficiently delivers hot water but also allows the heat pump and solar system to operate more efficiently, as both benefit from a cooler return flow.

System Controller Can Be Remotely Monitored and Programmed

To ensure the most energy-efficient use of all heat suppliers, i.e., the geothermal heat pump, solar system, and eXm device, the VarCon380 system controller monitors the entire system, prioritizing the solar system. The parameterizable and self-learning controller logic optimally adapts heat generation to consumption behaviors. The controller also enables operational data logging and remote access, allowing remote monitoring or system reprogramming.

"The eXm system with the system controller as the 'brain' and the eXm machine as the 'heart' offers exactly the intelligence and efficiency that passive houses need today," says Morscher. "For us, this is the new standard in heating technology."

How the eXm Device Raises Temperature in a Passive House

The eXm machine, developed jointly by Varmeco and BMS-Energietechnik, creates optimal temperature stratification in the heat storage system. To this end, a single-stage water-to-water heat pump operates inside the device, designed for a large temperature spread of about 50 K in the buffer tank and capable of operating at a source temperature of 55°C or higher. During operation, the eXm device draws water at a medium temperature level from the storage system. Part of it passes the condenser of the machine, where it is heated before entering the hot part of the tank. The other part passes over the evaporator, and the cooled water is then directed to the cold storage.

In this way, the eXm device makes heat available at a low temperature level usable at a high temperature level, for example, for domestic hot water preparation. This benefits many systems with low-temperature heat, such as heating heat pumps, solar systems, combined solar and PV modules, waste heat utilization, and low-temperature local heating networks.

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Waste Heat Utilization and Heat Recovery - Raising Supply Temperature to a Usable Level

Applications in industry, commercial enterprises with refrigeration and air conditioning systems, or process cooling. In the food industry, waste heat is often available, e.g., from compressor refrigeration units. Due to the low system temperatures (around 40°C to 45°C), meaningful use for heating or process purposes is currently possible only with significant technical effort. In this case, the eXm machine offers a simple way to generate the necessary system temperatures, e.g., to use waste heat for domestic hot water preparation. Using the eXm machine saves resources and reduces operating costs.

Solar Energy - Increased Solar Yield

Application: Residential and commercial buildings of all types. The efficiency of solar collectors is highly dependent on the difference between the medium temperature in the collector and the ambient air temperature (collector heat losses). Additionally, diffuse radiation does not achieve high system temperatures. The eXm device increases collector efficiency and solar yield, enables meaningful use of solar energy even with unfavorable inclination or east/west orientation, and makes costly preheating stages unnecessary.

Hybrid PVT Collectors - Increased Efficiency

Application: Residential and commercial buildings of all types. Photovoltaic modules generate electricity under light exposure, but as the module temperature rises, its electrical efficiency decreases. Hybrid PVT collectors dissipate heat that is undesirable during electricity generation, thus increasing electrical efficiency. Typical operating temperatures range from 20°C to 40°C. The eXm machine lowers return temperatures (used for cooling PVT modules) and raises low supply temperatures to a usable temperature level.

• Increases collector efficiency by up to 10 percent
• Supports heating - Boiler can be turned off more often
• Smaller collector surface area for desired performance
• Reduction of preheating stages
• Parallel electricity generation
• Better cooling of PV modules, thus higher electricity yield

BHKW - Gas Cogenerators

In cogenerators, the use of the eXm machine lowers the return temperature, making the water returning to the engine cooler. This increases heat generation efficiency, ensures proper engine or oil circuit cooling, and extends switching cycles.

• Lower return temperatures
• More effective engine cooling
• Increased system efficiency
• Fewer starts and stops, longer operating time
• Lower maintenance costs

District Heating Networks - Lower Return Temperature and Comply with Operating Conditions

In residential and commercial properties, hospitals, care facilities, training centers, etc.
Many district heating network operators associate connection with the condition that the return from the heating station occurs at a significantly lower temperature (e.g., 50°C or lower) than the flow. Especially in summer, when heat demand is low, this condition is difficult to meet—in the worst case, penalties may be imposed by the district heating operator. With the eXm device, a return temperature of < 40°C can be ensured regardless of usage (even in hot water-only mode and during circulation phases, i.e., times without hot water draw-off).
The eXm heat engine increases the difference between supply and return temperatures. This allows the return temperature to be lowered, ensuring compliance with operating conditions. The heat engine can maintain the target temperature.

Exergetically Optimized Heat Pumps

Modern heating systems should be reliable, efficient, resource-saving, and cost-effective to operate. eXm®-compact supports many different heat generators in this task.

• It ensures that heat pumps operate at their optimal operating condition and avoids unfavorable operating conditions with high outlet temperatures. This increases the annual performance factor of the entire system.
• In many cases, it enables the use of waste heat (typically at 30–40°C) in the first place, thus replacing fossil fuels. Additionally, there is a double benefit, as the waste heat used in this way would otherwise have to be dissipated using electrical energy.

Heat Storage in Buffers Is Important

Due to the buffer volume and the high heat storage capacity of water, it is possible to design heating system components to provide a large amount of heat for required peak loads with low power but long operating times.
This results in long operating and pause times for heat sources, such as heat pumps, avoiding cyclic behavior with corresponding unfavorable operating conditions.

Integration with Existing Heating Systems

eXm®-compact is designed to be easily integrated into existing heating systems with buffer tanks. Only four connections (sometimes only three) to the buffer tank and a 230 V power supply are needed—nothing more.

• eXm®-compact operates autonomously and requires no interference with the existing control system.
• Due to relatively low volumetric flow rates, there are no special requirements for piping or stratification devices in buffer tanks.
• Thanks to an external activation contact, eXm®-compact is also ready for smart grid operation.

Increases Supply Temperature, Shortens Switching Cycles, and Extends Operating Time

In all types of residential and commercial buildings, most heat pumps (air/water) typically deliver supply temperatures up to 50°C. This is sufficient for surface heating but insufficient for hygienic domestic hot water preparation. The eXm heat engine raises the temperature to 65°C or higher, making the heat usable for high-temperature heating or domestic hot water preparation. There is no need for additional heating with electric heaters or gas boilers.
Operating and pause times are extended, meaning the heat pump is switched on much less frequently.
On cold winter days, the heat pump can operate at lower temperatures, thus more efficiently.

Advantages:

• Hygienic domestic hot water preparation, as there is no possibility of storing drinking water.
• Short installation times due to factory configuration or installation.
• Simplified planning and installation thanks to perfectly matched hydraulics.
• Unique, integrated control with a self-learning algorithm.
• Increased annual system performance factor (COP).

Energy-Saving eXm Solutions Compared to Standard Solutions

A: Efficient Solution: Heat Pump + Fresh Water Domestic Hot Water System

VARIO fresh-nova Fresh Water Heater

Fresh water heaters heat fresh, cold drinking water in a continuous flow system. This avoids stagnation of hot water and minimizes the risk of Legionella growth. Fresh water heaters draw energy for water heating from the upper, hot area of the stratified tank. When hot water is drawn, heating water flows through a heat exchanger and transfers heat in counterflow to the passing fresh drinking water, with both water streams not mixing. This way, hot water is produced practically "just in time" while maintaining maximum hygiene. Unlike undersized domestic hot water tanks or heat exchangers, operation with a smaller temperature increase in the heating water is possible. This contributes to increasing the efficiency of the heat pump system.

• Energetically and exergetically better than a standard tank
• Advantages of Varmeco fresh water technology - fresh water system

 

B: More Efficient Solution: Heat Pump + Fresh Water Domestic Hot Water System + eXergy BackPack Buffer

eXergy Storage BackPack

This buffer tank is a stratified tank, meaning different temperature levels can be achieved—cool at the bottom and increasingly warmer toward the top. Thanks to connection sleeves and channels in the rear layer, heat generators and consumers can be connected at different temperature levels. In addition to decoupling efficiency, this also achieves advantageous hydraulic separation. High-quality thermal insulation minimizes losses and ensures long-term availability of stored energy. eXergy BackPack tanks—regardless of size—are designed for direct mounting of the VARIO fresh-nova fresh water heater. Simply attach, connect, and it’s ready.

• Fastest installation time
• Coordinated hydraulics (error prevention)

 

C: High-Efficiency Solution: Heat Pump + Fresh Water Domestic Hot Water System + eXergy BackPack Buffer + eXergy Machine

eXergiemaschine-compact, or eXm-compact

eXm-compact is a kind of "add-on" to the heat pump and ensures a wide temperature spread in the eXergy tank. During operation, eXm-compact draws water from the middle of the stratified tank, where the heat pump for heating is typically connected. Part of this water is heated before entering the upper, hot part of the tank; the other part is cooled and directed to the lowest, cold part of the tank. This happens autonomously and independently of the current user behavior, i.e., the demand for heating or hot water. This achieves high supply temperatures for consumers, such as domestic hot water heating, and defined low return temperatures to the source—ensuring greater yield for solar systems and an optimal operating condition for the heat pump.

• Increases overall system efficiency
• Makes useless heat usable
• Enables domestic hot water preparation at low heat pump system temperatures

ANNUAL PERFORMANCE FACTOR (JAZ - Jahresarbeitszahl)

• Brine/water heat pump + eXm-compact = 4.5–6 JAZ - Annual performance factor of the entire heating system
• Brine/water heat pump + electric heater = only 2–3.8 JAZ
• High-temperature brine/water heat pump (up to 65°C) = 3.2–4.1 JAZ
• Air/water heat pump + eXm-compact = 4–5.3 JAZ
• Air/water heat pump + electric heater = only 1.5–3.2 JAZ
• High-temperature air/water heat pump (up to 65°C) = only 1.8–3.5 JAZ

Is COP the Same as JAZ?

No, COP is not the same as JAZ, but it is absolutely essential for calculating the annual performance factor. The coefficient of performance (COP) is a performance number that expresses the ratio of usable heat to consumed electrical energy at specific operating conditions. What does this mean exactly? COP is a snapshot value under specific standard conditions and refers only to the heat pump, not the entire heating system.
In simplified terms, the seasonal performance factor is the ratio of generated heat to the electrical energy required for it, which can be expressed by the formula below:
JAZ = heating energy (kWh/a) / electrical energy (kWh/a).
Both values refer to a full year (a = anno), resulting in the annual performance factor.

Cold District Heating Networks

Central Domestic Hot Water Heating

Decentralized Domestic Hot Water Heating

Waste Heat Utilization and Heat Recovery

Raising Supply Temperature to a Usable Temperature Level

In industry, commercial facilities with refrigeration and air conditioning systems, or process cooling, refrigeration systems, e.g., for operating cold stores and freezers, generate waste heat at a temperature level of 30°C to 40°C. This is too low to be used for heating or domestic hot water preparation!
The eXergy machine can raise the heat to a higher supply temperature. This means a usable temperature level can be achieved and is available for process heating, space heating, or domestic hot water preparation.
With lower return temperatures, waste heat can be utilized even more effectively. The use of eXm saves resources and helps reduce operating costs.

Solar Energy - Increase Solar Energy Yield

In residential and commercial buildings, a low return temperature positively impacts solar energy yield. In transitional and winter months, solar heat can be used more efficiently and for longer at a lower level.

Case Study eXm - Multi-Family Building

Multi-Family Building with Air-to-Water Heat Pump and Central Domestic Hot Water Preparation

• Heating load: 30 kW (50,500 kWh/year)
• Underfloor heating with supply/return temperature: 35/28°C
• 18 people, domestic hot water demand: 40 l/person*day at 60°C
• Circulation losses: 10%
• Total heat demand: 76,500 kWh/year

Simulated Variants

• Heat pump
• Heat pump + 30 m² solar thermal
• Heat pump + eXm
• Heat pump + 30 m² solar thermal + eXm

Diagram: Air-to-Water Heat Pump + Solar Thermal

Boundary conditions:
The air-to-water heat pump serves underfloor heating and heats the buffer to 55°C; then a 9 kW electric heater heats the upper part of the buffer to 63°C for domestic hot water preparation. This also applies to systems without a solar installation.

Diagram: Air-to-Water Heat Pump + eXm + Solar Thermal

Boundary conditions:
The air-to-water heat pump operates continuously at a supply temperature of 35°C; eXm transfers heat from the low-temperature tank to the high-temperature tank as needed, with a set supply temperature in the HT circuit at 64°C, and the solar installation has a switching valve in the supply circuit. Boundary conditions also apply to systems without a solar installation.

Comparison of Variants: Electricity Consumption

Results - Electricity Demand, Non-Renewable Primary Energy Input Factor, Annual System Performance Factor:

Variant	Energy Consumption (kWh/year)	AAZ (Input Factor)	SJAZ (Annual Performance Factor)
Heat Pump	31,187 kWh/year	AAZ: 0.74	SJAZ: 2.4
Heat Pump + Solar	27,622 kWh/year	AAZ: 0.69	SJAZ: 2.7
Heat Pump + eXm	24,673 kWh/year	AAZ: 0.59	SJAZ: 3.1
Heat Pump + eXm + Solar	21,240 kWh/year	AAZ: 0.50	SJAZ: 3.6

Savings in %; CO2 Emissions (666 g/kWh)

1. Air-to-Water Heat Pump