Author: Mycond Technical Department
Humidity control in buildings has a significant impact on overall greenhouse gas emissions. A proper understanding of the principles behind the carbon footprint of different dehumidification technologies enables informed engineering decisions that substantially reduce environmental impact. This method proposes a structured approach to assessing and reducing CO₂ emissions from dehumidification systems, taking into account both direct energy consumption and the indirect impact on the building’s overall heating, ventilation and air conditioning (HVAC) system.
The thermodynamic nature of CO₂ emissions in moisture removal processes
At the core of dehumidification lies moisture removal, which requires energy proportional to the latent heat of vaporisation of water. This physical quantity depends on temperature according to: latent heat of vaporisation = 2501 - 2.38 × temperature (kJ/kg), where temperature is measured in degrees Celsius. At typical indoor temperatures (+20°C), evaporating 1 kg of water requires about 2453 kJ/kg of energy.
Different dehumidification technologies realise this process differently, as shown on the enthalpy–humidity ratio psychrometric chart. In condensation dehumidification, air is cooled below the dew point; in adsorption dehumidification, water vapour is bound by an adsorbent; and in ventilation dehumidification, moist indoor air is replaced by drier outdoor air.
The conversion of energy into CO₂ emissions occurs via the primary energy conversion factor (typically from 2.0 to 3.0 for the electricity grid, and from 1.1 to 1.3 for gas) and the carbon intensity of electricity, measured in grams of CO₂ per kilowatt-hour. These values depend on the regional generation mix and can vary significantly.
It is important to distinguish between the dehumidifier’s direct energy consumption and its indirect impact on the main HVAC system. Ignoring the impact on chillers and boilers leads to a 40–80% error in assessing real emissions, since the heat released during dehumidification adds cooling load in summer or reduces heating load in winter.
Energy and carbon profile of condensation dehumidification
Condensation dehumidification is based on the thermodynamic cycle of a refrigeration machine, where air is cooled on the evaporator below the dew point to condense moisture and then heated on the condenser. The coefficient of performance (COP) depends on air temperature and for the range from +5 to +35°C typically varies from 1.5 to 3.5 — the higher the air temperature, the higher the efficiency.
The specific energy consumption of a condensation dehumidifier is calculated as the ratio of electrical power to moisture removal rate (E(specific) = P(electric) / G(moisture)), and typically ranges from 0.3 to 0.7 kWh/kg of moisture depending on operating conditions.
When a condensation dehumidifier operates, all electrical energy is converted into heat and released into the space together with the heat of condensation of moisture. The total heat released equals the sum of the latent heat and the electrical power (Q(condenser) = G(moisture) × r + P(electric)), which in summer creates additional load on the building’s cooling system, increasing indirect emissions.
In addition to indirect emissions, condensation dehumidifiers have direct emissions from refrigerant losses, calculated as the mass of losses multiplied by the refrigerant’s global warming potential (GWP). Current trends focus on replacing high-GWP refrigerants with alternatives with a lower climate impact.
Energy and carbon profile of adsorption dehumidification
Adsorption dehumidification works on the principle of physical or chemical adsorption of moisture by special materials (silica gel, zeolite, LiCl, etc.). The process consists of two phases: adsorption, when the material absorbs moisture by lowering the partial pressure of water vapour, and regeneration, when the adsorbent is heated to high temperature (120–180°C) to remove the absorbed moisture.
The specific energy consumption of an adsorption dehumidifier is determined mainly by the energy required for regeneration, and depends on heat recovery effectiveness, regeneration temperature and adsorbent type. Energy consumption is calculated using a formula that accounts for air heating, heat of desorption and heat recovery effectiveness.
Energy sources for regeneration may vary: electric heaters, gas burners, hot water or steam. Each source has its own carbon intensity, which significantly affects the overall carbon footprint. Gas-fired regeneration usually has a lower carbon footprint in regions with high electricity carbon intensity.
It is important to consider additional fan load due to increased pressure drop in the adsorber, as well as the potential for heat recovery downstream of the adsorber or additional load on the building’s cooling system.
Energy and carbon profile of ventilation dehumidification
Ventilation dehumidification is based on the psychrometric principle of replacing indoor air with outdoor air, provided the outdoor moisture content is lower than indoors. This is the most energy-efficient method, but its availability is limited by climatic conditions.
Climatic availability is evaluated by analysing hourly meteorological data and determining the share of hours per year when the outdoor moisture content is lower than the required indoor level. In some climate zones of the United Kingdom this method is available for more than 6000 hours per year.
The energy consumption of ventilation dehumidification is associated with the thermal treatment of supply air — heating during the heating season and cooling in summer. Applying heat recovery with an effectiveness from 0.5 to 0.85 significantly reduces this energy use.
Algorithm for selecting a technology based on minimal CO₂ emissions
The choice of dehumidification technology with the lowest carbon footprint should be based on a structured approach:
- Determine the annual moisture removal from the facility’s moisture balance.
- Calculate the specific energy consumption for each dehumidification technology, considering real operating conditions.
- Account for the impact on the main HVAC system — additional cooling load or reduced heating load.
- Multiply energy consumption by the primary energy conversion factor and carbon intensity to determine indirect emissions.
- Add direct refrigerant emissions for condensation systems.
- Compare total emissions for different technologies.
General boundary conditions for technology selection:
- If air temperature is below 15°C, adsorption dehumidification has an advantage.
- If outdoor moisture content is lower than indoor for more than 4000 hours per year, ventilation dehumidification has an advantage.
- If there is a sink for low-grade heat, condensation dehumidification with heat recovery has an advantage.
Recovery of condensation heat: calculating the emissions reduction potential
There is significant potential to reduce CO₂ emissions when using condensation dehumidification due to the possibility of recovering condensation heat. The maximum heat available for recovery is calculated as the product of moisture removal rate and latent heat of vaporisation plus electrical power: Q(recovery) = G(moisture) × r + P(electric).
This heat can be used for various low-grade consumers:
- Domestic hot water heating to 50–60°C
- Swimming pool water heating to 26–28°C
- Air heating to 35–50°C
- Process applications with appropriate temperature requirements
The temperature potential of recovery is determined by the refrigerant condensing temperature, which typically ranges from 40 to 55°C for dehumidification systems operating at an indoor temperature of around +20°C. Heat exchanger effectiveness is limited by the minimum temperature approach between fluids, usually 3 to 5 kelvins.
Methodology for calculating the full carbon footprint of a dehumidification system: the TEWI method
The TEWI (Total Equivalent Warming Impact) method enables a comprehensive assessment of the climate impact of condensation dehumidification systems. TEWI is calculated as the sum of three components:
TEWI = GWP × M(leakage) × L + GWP × M(charge) × (1 - α(recovery)) + E(annual) × L × β × PEFC
where:
- GWP — global warming potential of the refrigerant
- M(leakage) — annual refrigerant losses
- L — equipment service life in years
- M(charge) — refrigerant charge mass
- α(recovery) — refrigerant recovery fraction at end-of-life
- E(annual) — annual energy consumption
- β — electricity carbon intensity
- PEFC — primary energy conversion factor
For adsorption systems, the method is modified to reflect the absence of refrigerant, while including the various energy sources used for regeneration. To compare different technologies, results are normalised to a common basis — kilograms of CO₂ equivalent per kilogram of moisture removed per year, or per square metre of area per year.
Integration with renewable energy sources: calculating carbon footprint reduction
Significant reductions in the carbon footprint of dehumidification systems are possible through integration with renewable energy sources. For adsorption systems, using heat pumps for adsorbent regeneration is effective, with a coefficient of performance from 2.0 to 3.5 for regeneration temperatures from 120 to 140°C.
Solar collectors for adsorbent regeneration require calculation of the necessary area using: A(collectors) = Q(regeneration) / (I(insolation) × η(collector) × k(utilisation)), where collector efficiency ranges from 0.4 to 0.7 depending on type.
For condensation dehumidifiers, the use of photovoltaic systems is promising, with the load coverage fraction calculated as: k(coverage) = P(PV) × T(generation) / E(annual). Battery energy storage can effectively smooth peak demand, but its feasibility depends on the operating regime of the dehumidification system.
Impact of grid carbon intensity on technology selection
The carbon intensity of electricity is a critical factor when selecting a dehumidification technology. It varies significantly from region to region: from 50 g CO₂/kWh (Norway, Sweden) to 800 g CO₂/kWh (Poland). In the United Kingdom, the average value is about 250 g CO₂/kWh, but it is constantly changing with a downward trend.
Official data from system operators are used to determine carbon intensity. When comparing dehumidification technologies across countries, results can differ markedly. For example, a condensation dehumidifier with a COP of 2.5 may have a lower carbon footprint than an adsorption unit with gas regeneration at an electricity carbon intensity below 400 g CO₂/kWh, but will be worse at higher values.
Regulatory requirements and building environmental certification systems
The Energy Performance of Buildings Directive (EPBD) sets requirements for nearly zero-energy buildings (nZEB), which are gradually becoming mandatory in the EU and the United Kingdom. The F-gas Regulation 517/2014 imposes restrictions on refrigerants with a global warming potential above 2500 (from 2020) and above 150 (from 2025).
Building environmental certification systems such as BREEAM, LEED and DGNB include criteria for evaluating energy efficiency and emissions, where the TEWI method is used for a comprehensive assessment of dehumidification systems. Regulatory trends include banning high-GWP refrigerants, mandating the use of renewables and introducing carbon pricing.
Common engineering mistakes and misconceptions
The most common mistakes when assessing the carbon footprint of dehumidification systems are:
- Comparing technologies solely by direct energy consumption, ignoring the impact on the building’s HVAC system
- Applying a universal carbon intensity value without accounting for the local generation mix (error up to 400%)
- Ignoring direct refrigerant emissions
- Overestimating heat recovery potential without calculating a real sink and temperature matching
- Assessing renewables by installed capacity without calculating the utilisation factor
- Comparing adsorption dehumidification with electric regeneration rather than gas-fired regeneration
- Failing to account for performance degradation over the service life
- Ignoring embodied carbon from equipment manufacturing
Boundaries of applicability of the methods and conditions of ineffectiveness
When choosing a dehumidification technology, it is important to consider their limitations:
- Temperature limits of condensation dehumidification: at temperatures below +5°C, a COP below 1.5 makes the method uneconomic
- Limits of the ventilation method: effective only when outdoor moisture content is lower than indoors; impossible in humid climates
- Project scale for heat recovery: at capacities below 50 kg/day, capital expenditure on heat recovery is often unjustified
- Climatic constraints for solar regeneration: in Northern Europe and the United Kingdom, especially at latitudes above 55°, insolation below 1 kWh/m² per day in winter results in load coverage below 20%
Frequently asked questions
How do I determine which dehumidification technology will have the lowest carbon footprint for my facility?
You need to assess the facility’s annual moisture balance, climatic conditions (temperature and moisture content of outdoor air), the carbon intensity of available energy sources and the potential to utilise recovered heat. Then, using the TEWI method, calculate the total equivalent warming impact for each technology, accounting for the impact on the main HVAC system.
Is condensation dehumidification always less efficient in terms of CO₂ emissions?
No, under certain conditions condensation dehumidification can have a lower carbon footprint. This occurs when: 1) air temperature is high (above 20°C), providing a high coefficient of performance; 2) there is a low-grade heat sink for recovery; 3) the carbon intensity of electricity is low (below 300 g CO₂/kWh).
How should the impact of a dehumidification system on the overall HVAC system be assessed correctly?
For condensation dehumidification, calculate the heat released into the space as the sum of the heat of condensation of moisture and the electrical power. This heat creates additional load on the cooling system in summer, or reduces the heating load in winter. For adsorption dehumidification, account for the heat released during adsorption of moisture and the potential increase in air temperature.
What is the typical payback period for heat recovery systems on condensation dehumidifiers?
The payback period is typically 2 to 7 years, depending on project scale, operating regime, energy prices and the availability of a continuous heat sink. The best payback (2–3 years) is achieved for high-capacity systems (over 100 kg/day) with continuous operation and a heat sink with temperature requirements below the condensing temperature.
Which dehumidification technology is best suited for integration with renewable energy sources in the United Kingdom?
In the United Kingdom, the most promising approach is to combine ventilation dehumidification with heat recovery for the heating season (which constitutes most of the year) and condensation dehumidification powered by photovoltaic panels for the summer period. Adsorption dehumidification with heat pump regeneration can also be effective for facilities that require a consistently low humidity level regardless of outdoor conditions.
Conclusions
A comprehensive assessment of the carbon footprint of dehumidification systems requires broadening the analysis beyond the equipment’s direct energy consumption. The determining factors are:
- The impact on the building’s main HVAC system, which can change total emissions by 40–80%
- The carbon intensity of the energy sources used, which varies greatly by geography and over time
- The potential for integration with renewable energy sources and heat recovery
- Climatic conditions, which determine the effectiveness of different technologies throughout the year
The algorithm for selecting a dehumidification technology with the lowest carbon footprint should include a comprehensive TEWI assessment, accounting for both direct and indirect emissions over the entire life cycle of the system. Calculations should consider seasonal efficiency variations, equipment performance degradation over time and the projected decarbonisation of the energy sector.
The tightening of regulatory requirements and the development of building environmental certification systems make accounting for the carbon footprint of dehumidification systems not only environmentally responsible, but also an economically justified decision for current and future projects.
