DOMESTIC DEHUMIDIFIERS IN COOL CONDITIONS: I PERFORMANCE FACTORS

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1 DOMESTIC DEHUMIDIFIERS IN COOL CONDITIONS: I PERFORMANCE FACTORS MJ Cunningham 1 and CG Carrington 2 1 Building Research Association of New Zealand, Moonshine Rd., Porirua City, New Zealand 2 Department of Physics, University of Otago,Dunedin, New Zealand ABSTRACT 25% of New Zealand homes use s. Most non-living s in New Zealand houses are unheated with temperatures frequently below 1 o C, which means that these s are operating in a very inefficient part of their performance envelope. Their performance under these conditions is modelled. It has been found that the s operate in two different regimes: in the ventilation-enhanced regime extra ventilation of outdoor air helps lower indoor humidities; while in the ventilation-hindered regime extra ventilation tends to raise indoor humidities. Performance is modelled as a function of type, strength of moisture sources, level of heating, ventilation, insulation level and external climate. INTRODUCTION 25% of New Zealand homes have portable s (Isaacs et al 24) used by house occupants seeking a quick and easy solution to window condensation problems, mould growth and other symptoms of dampness. Domestic s were designed, in the first instance, to work in warm and humid climates: so, for example the Association of Home Appliance Manufacturers (AHAM, 1992) test capacity at 26.7 o C, maximum operating conditions at 32.2 o C, and low-temperature test at 18.3 o C. In New Zealand s are used primarily in cold beds where the temperature may be 1 o C or lower (Cunningham et al 24). It is quite unclear how well the s perform in practice under these colder conditions. RESEARCH METHODS The mass balance equation for a in a is: V c t = F A A outdoors hygro window V ( p ( p c outdoors p p surface + F window house V ) r + S ) r S c house occupants + S ( F sources outdoors + F house ) V c (1) where V (m 3 ) is a volume in zone specified by its subscript; c (kg/m 3 ) is a moisture concentration in zone specified by its subscript; F (s -1 ) is an air change rate in zone specified by its subscript; A hygro (m 2 ) is the exposed surface area of the hygroscopic contents; p (Pa) is a vapour pressure in zone specified by its subscript; S (kg/s) is a moisture release rate (or uptake rate if negative) from a source (or sink) specified by its subscript; r (m/s) is the vapour resistance of the boundary layer above the hygroscopic materials; ρ (J/kg o C) is the specific heat of air at constant pressure. The window moisture transfer term is invoked if the window is below the dewpoint or has condensation on it. For moisture transfer within the s hygroscopic materials we have the 2 m m diffusion equation, = D where m (kg/m 3 ) is the moisture content within the material and D (m 2 /s) is 2 t x its diffusion coefficient. A similar pair of equations can be written down for the heat balance. The above set of equations is solved numerically using an explicit finite-difference algorithm. Data Used For Model Calculations Unless otherwise specified the conditions used in this paper are: the is assumed to operate in a that has a floor plan of 3 metres by 4 metres, a 2.8 metre stud, and 8 m 2 of single glazing. 1

2 the is uninsulated with a UA value of 124 m 2 o C /W. the external climate is modelled with a daily sinusoidal, specifically. T = 9 + 6sin( 2π ( t + 18) / 24) and ϕ = sin(2π ( t + 6) / 24) where t is the time in hours; T is the temperature and ϕ is the relative humidity the is humidistated to % relative humidity. (Note this does not necessarily imply that % RH is achieved very often the is not able to pull the relative humidity down to this level). the has one occupant at rest providing Watts of heat and outputting ml of moisture per hour. The ventilation rate is.5 air changes per hour (ach). RESULTS AND DISCUSSION Two operating regimes By differentiating the steady state equation with respect to F it can be shown that: dc < H < S coutdoors < c df which says that if the takes moisture out of the air at a lesser rate than it is being generated internally, then: (1) the indoor air moisture concentration is greater than the outdoor moisture concentration; and (2) the indoor air moisture content decreases if the ventilation rate is increased. This regime is called Ventilation-enhanced in this work, see Figure 1. Conversely if H S we have coutdoors c i.e. if the takes moisture out of the air at a greater than rate than it is being generated internally, then: (1) the indoor air moisture concentration is less than the outdoor moisture concentration; and (2) the indoor air moisture content increases if the ventilation rate is increased. This regime is called Ventilation-hindered, see Figure Relative humidities acheived in ventilation-enhanced and ventilation-hindered as a function of ventilation Ventilation rates (ach) Ventilation-enhanced. (9 o C mean outdoor temperature, 8% mean outdoor relative humidity) Ventilation-hindered. (22 o C mean outdoor temperature, 85% mean outdoor RH) Figure 1. Achieved relative humidities in ventilation-hindered and ventilation-enhanced regimes Factors that determine performance 11

3 1 An example of actual perfomance when humidistated to % Humidity (%) Time (hours) With humidifier Without Figure 2. Humidity achieved when a is humidistated to % rh. Performance in General A humidistated to say % relative humidity may achieve relative humidities above % if moisture sources overwhelm the, or below % if natural conditions fall to those levels and the turns itself off, see Figure 2. Group A 2 23 watts C 15 C 2 C RH % Figure 3. Group A, average input power Group A ml/hour C 15 C 2 C RH % Figure 4. Group A, average drying rate Dehumidifier type Domestic s available in New Zealand can be classified by decreasing performance into 3 groups (Carrington 23). In this work we report just the group, labelled Group A, with the highest average energy efficiency. Figure 3 and Figure 4 show the performance of this group of 12

4 s, Carrington (23). These curves are parameterised to provide a formula for the term S in equation (1) for use in the numerical model. Relative humidity achieved by a Group A humidistated to % RH as a function of the number of people in the 75 Relative humidity acheived (%) UA = 124 W/oC Number of people UA = 56.1 W/oC Figure 5. Effect of moisture sources on performance Effect of moisture source. Figure 5 shows the modelled mean relative humidity achieved by a Group A humidistated to % RH as a function of the number of people in the, i.e. the size of the moisture source. 65 Relative humidity achieved with a Group A at different heating levels Relative humidity (%) 55 UA = 124 W/oC UA = 56.1 W/oC UA =.4 W/oC Heating level (W) Figure 6. Effect of heating. Mean relative humidity achieved by a humidistated Group A at different heating levels. Effect of Heating Heating raises the temperature of the, thereby lowering relative humidities. Figure 6 shows how this affects performance for a humidistated Group A at different heating levels. Heating beyond W at the lower UA values results in temperatures greater than 28 o C. Effect of Ventilation Whether ventilation improves or makes worse the performance a depends upon whether it is operating in a ventilation-hindered or ventilation-enhanced regime, see Figure 7 above and Figure 1. Effect of Insulation Figure 7 also shows the effect of changing insulation levels on performance. As expected performance improves as insulation levels rise (lower UA) Effect of external climate Figure 8 shows the mean relative humidity achieved by a humidistated (to %) Group A as a function of degree-days. The drop off in performance as the climate gets colder, and at lower insulation levels is quite clear. 13

5 7 Effect of insulation levels. Group A humidistated. Mean RH achieved (%) ach.5 ach 1 ach 2 ach 5 ach UA level (W/oC) Figure 7. Effect of insulation levels Performance of a Group A by degree-days Kaitaia Auckland Hamilton Wellington Dunedin Christchurch Invercargill UA = 124 W/oC UA = 56.1 W/oC Degree-days (oc d) Figure 8. Mean relative humidity achieved by a humidistated (to %) Group A as a function of degree-days CONCLUSIONS AND IMPLICATIONS Under the colder conditions often found in for significant periods of time in many New Zealand houses the modelling shows that s are not able to pull the relative humidity down to the humidistated setpoint, see for example Figure 2. Higher temperatures are required to shift the s into a part of their performance curves where they operate more effectively. These temperatures appear in insulated well heated houses where it should not be necessary to use s. The s were found to operate in two regimes, called the ventilation-enhanced and ventilation-hindered regimes. The ventilation-hindered regime occurs when the pulls the indoor absolute humidity below the outdoor absolute humidity. Under these conditions additional ventilation of outdoor air increases the indoor humidity. REFERENCES AHAM ANSI/AHAM Standard DH , American National Standard Dehumidifiers, Chicago: Association of Home Appliance Manufacturers. Carrington CG. 23. Performance Models for Domestic Dehumidifiers Report for the Building Research Association New Zealand, Physics Department, Otago University (New Zealand), 2 pages. Cunningham M, Viggers H, Matheson A, Howden-Chapman P. Changes of exposure to low temperatures and high humidities on retrofitting houses with insulation. 2nd WHO International Housing and Health Symposium September 29 - October 1, 24, Vilnius, Lithuania. Isaacs NP, Amitrano L, Camilleri M, French L, Pollard A, Saville-Smith K, Fraser R. and Rossouw P. 24. Energy Use in New Zealand Households: Report on the Year 8 Analysis for the Household Energy End-use Project. BRANZ, Porirua (New Zealand) 14

6 DOMESTIC DEHUMIDIFIERS IN COOL CONDITIONS II COMPARISON WITH OTHER METHODS OF RELATIVE HUMIDITY CONTROL M J Cunningham 1 1 Building Research Association of New Zealand Moonshine Rd., Porirua City, New Zealand ABSTRACT A companion paper outlines the performance of domestic s in the cool New Zealand climate. This paper compares the energy cost of achieving dehumidification by alternative means, viz. more ventilation, more heating or more insulation. In all conditions examined all strategies to control relative humidity without a use much more energy than that of controlling indoor relative humidities by a alone. INTRODUCTION A companion paper modelled the performance of domestic s in cool New Zealand climates. (Cunningham and Carrington 25). The actual performance of the s was seen to be quite complex but in general s are not able to pull the relative humidity down to a humidistated setpoint all the time. Since the s are operating in a low efficiency part of their performance envelope it is unclear whether the use of s is cost- or energy- efficient in these cool conditions. To address this question this paper compares the energy consumption of s with the energy consumed if dehumidification is achieved by other means, specifically by more ventilation, more heating or more insulation. RESEARCH METHODS Numerical Model A description of a numerical model of the performance of a domestic in a is described in a companion paper, Cunningham and Carrington (25). The model is based on a heat and moisture balance within a accounting for: ventilation in and out of the ; possible condensation on cold surfaces; moisture sources; and the extraction rate of the. Once the model showed time variation of the humidity achieved at given driving conditions each of the alternative strategies was required to reproduce exactly, moment by moment, the relative humidity achieved by the. This of course is an artificial goal but it provides the only way to get a direct and robust assessment of the merits of the alternative strategies to control relative humidities. On the other hand temperatures were allowed to float freely to whatever level resulted. Dehumidifier and alternative-strategy energy consumptions were calculated and compared. Only Group A s, Cunningham and Carrington (25), are considered in this paper. The driving conditions and parameters are given in the companion paper, Cunningham and Carrington (25). Strategy Of More Ventilation In general it is not possible to reproduce the entire performance of a by ventilation alone. Anytime a is working effectively enough to pull the indoor air moisture content (absolute humidity) below the outdoor air moisture content ventilation causes the indoor humidity to rise, not fall. In this case the is operating in the ventilation-hindered regime, Cunningham and Carrington (25). A more realistic strategy to reproduce the same performance as a is to use ventilation at a certain level, and to supplement that with heating whenever the ventilation level is inadequate, or whenever ventilation is counterproductive, to bring the indoor relative humidity down to the required level. This is the strategy of opening the windows and heating if necessary. Figure 9 shows the mean of the temperatures required to achieve the required relative humidities at each level of enhanced ventilation for UA = 124 W/ o C, while Figure 1 shows the daily supplementary heating energy 1

7 consumption required to achieve these temperatures. Since the actual relative humidity achieved by the varies in a complex way, the temperatures must also vary in a similarly complex way about their mean to achieve the stated goal of reproducing exactly, moment by moment, the relative humidity achieved by the. 22 Extra ventilation and supplementary heating. Temperatures that need to be generated. UA=124 W/oC 2 Temperature (oc) ach without 1. ach without 1.5 ach without 2. ach without Ventilation level with (ach) Figure 9. Strategy of providing extra ventilation and supplementary heating. Mean of the temperatures required at each level of enhanced ventilation. UA = 124 W/oC 7 Extra ventilation and supplementary heating. UA=124 W/oC Daily energy consumption (kwh) Ventilation level with (ach).5 ach without 1. ach without 1.5 ach without 2. ach without Dehumidifier only Figure 1. Daily energy consumption required at each level of enhanced ventilation to generate the temperatures shown in Figure 9. It can be clearly seen that the heating energy required for this strategy is many times higher than that needed by a if controlling relative humidity with the alone. Strategy Of More Heating Indoor relative humidity can be lowered to any desired level simply by increasing the indoor temperature. Whether heating uses less energy than dehumidifying and whether this results in comfortable indoor temperatures is examined here. 11

8 8 Temperature required to reproduce the humidity control of a humidifier 1 Temperature (oc) ` RH (%) Temperature required Temperature with Outdoor air temperature Humidity achieved Outdoor humidity Time (hrs) Figure 11. Strategy of providing extra heat. Temperature required to reproduce the humidity control UA = 124 W oc-1, (uninsulated) mean outdoor conditions 9 oc, 8% RH. Figure 11 and Figure 12 show the performance for a with a UA value of 124 W o C -1 (poorly insulated). An average of 222W (53.4 kwh daily energy consumption) of heating power is required to give the same performance as a Group A, which requires only an average power level of 18 W (4.32 kwh daily energy consumption). If heating is used to control humidity (to exactly the same levels as a Group A humidistated would achieve) the average temperature required is 18.9 o C with a peak of 23.5 o C. 3 2 Extra heating power required to reproduce the humidity control of a humidifier Power (W) Time (hrs) Extra heating power required Dehumidifier power consumption (W) Figure 12. Strategy of providing extra heat. Extra heating power required to reproduce the humidity control of a Group A humidistated to % RH. Figure 13 shows daily heating energy consumption over the range of UA values to reproduce the performance of a Group A, humidistated at % relative humidity. Plainly heating is an unattractive option from an energy consumption viewpoint across all realistic ranges of insulation levels and all climates considered. 12

9 Daily energy use (kwh) Daily energy use compared to daily energy using heating only, versus envelope UA value Alternative heating energy required per day (kwh). Average external temperature 9 oc UATotal (W/oC) Figure 13. Strategy of providing extra heat. Alternative heating energy required (kwh) per day. Average external temperature 18 oc Dehumidifier energy consumption per day (kwh) Average external temperature 9oC Dehumidifier energy consumption per day (kwh) Average external temperature 18oC Daily energy use of a Group A humidistated to % RH compared to daily energy use using heating only to achieve the same relative humidity, as a function of the envelope UA value, ventilation =.5 ach. Strategy Of More Insulation Increasing insulation levels alone is not enough to achieve relative humidity control at the same level as a can. Additional heating is also required. 3 Extra insulation and supplementary heating. UA =124 W/oC with Power (W) Dehumidifier only No. UA = 124 W/oC No. UA = 93. W/oC No. UA =62. W/oC Time (hours) Figure 14. Strategy of providing extra insulation and supplementary heating. Case of a Group A humidistated with UA = 124 W/ o C. Supplementary heating required at different insulation (UA) levels. Figure 14 considers the case of a humidistated Group A operating in a with UA = 124 W/ o C, with mean outdoor conditions of 9 o C, 8% RH. The average power level driving the is 18W giving a daily energy use of 4.32 kwh. To exactly reproduce the relative humidity control by heating alone requires a temperature regime with a mean temperature of 19. o C and a maximum temperature of 23.5 o C achieved with an average heating power of 2224W which is a daily energy use of 53.4 kwh. If the insulation level is increased by % so that UA = 93 W/ o C supplementary heating with an average power of 1614W is required which is a daily energy use of 38.7 kwh. If the insulation level is increased by 1% so that UA = 62 W/ o C supplementary heating with an average power of 13W is required which is a daily energy use of 24.1 kwh. 13

10 All cases use considerably more energy than that consumed by a alone achieving the same relative humidity levels. CONCLUSIONS AND IMPLICATIONS Alternative strategies were investigated and modelling done to reproduce exactly, moment by moment, the humidity produced by the, including reproducing relative humidities greater than the setpoint when the could not pull down to the setpoint, and reproducing humidities less than the setpoint when the humidistat had turned the off. When heating was used in an alternative dehumidifying strategy, warmth and comfort were not considered, except for excluding cases where peak indoor temperatures rose above 28 3 o C. In all conditions examined, chiefly for unheated poorly insulated s, and if heating for warmth and comfort is not under consideration, all alternative strategies to control relative humidity without a more ventilation (with supplementary heating), more heating, or higher insulation levels (with supplementary heating) use many times the energy of that used by a alone. Despite the fact that the s are operating in a low efficiency region of their performance envelope they are much more energy efficient in achieving lower humidities than other ways investigated here. REFERENCES Cunningham M. and Carrington G. Domestic Dehumidifiers in Cool Conditions I Performance Factors. Proceedings of Indoor Air 25, The 1th International Conference on Indoor Air Quality and Climate. 14