Physics of low energy storage - summary
Tim Padfield*, firstname.lastname@example.org
Morten Ryhl-Svendsen, Danish School of Conservation
Poul Klenz Larsen, National Museum of Denmark
Lars Aasbjerg Jensen, National Museum of Denmark
This article has been submitted (2017-10-20) for the 2018 conference of the International Institute for Conservation, in Turin: Preventive conservation: the state of the art
The need to keep things cool for durability should be the single most powerful influence on storage design. The simplest temperature control is to moderate the outside temperature by a combination of thermal insulation and heat capacity.
The low energy storage building is a lightweight, thermally insulated, airtight building put on top of an uninsulated floor slab laid directly on the ground. The thermal insulation is calculated to even out the daily temperature cycle but to allow an annual temperature cycle which is about half the amplitude, but much smoother, than the annual temperature cycle outside.
The winter temperature will nearly always be above ambient and so will maintain a moderate RH without need for either humidification or dehumidification. The temperature in summer will be below ambient and thus will force dehumidification. However, the airtightness of the building allows intermittent dehumidification with low energy consumption, less than one kWh/m3 per year.
There now exist enough buildings designed on this principle to reassure curators that highly valued collections can be stored in a space with a gentle temperature cycle that has never been demonstrated to cause damage and with a RH stability that is better than air conditioning usually achieves.
One important hindrance remains: curators and managers will not lightly go against the advice, or the instructions, of museum and archive standards, which continue to give precise limits to permitted temperature ranges.
There is abundant evidence for the beneficial effect of low temperature in increasing the durability of artefacts. The lowest temperature which can be achieved by simple means, using no energy at all, is the annual ambient average temperature. The areas on earth which have an average temperature below the human comfort zone (which lies above 20°C) comprise most of north America, Europe and Asia north of the Himalaya.
An economic and simple solution for museum and archive storage, is to allow a slowly varying temperature following the seasons, but reduced in amplitude. The summer temperature in the store will usually be below ambient. This will cause the relative humidity (RH) to rise above ambient. Summer dehumidification is necessary. The low point of the temperature cycle in winter can be adjusted to make humidification unnecessary. This storage principle requires a very large heat storage capacity and an airtight building. It also benefits from humidity buffering. This permits intermittent dehumidification, allowing direct powering from solar voltaic panels on the roof. All these considerations are gathered into the principle sketch, figure 1.
The building can be of lightweight construction, the vital requirement being a U-value (heat conductivity) around 0.1 W/m2·K. This value, combined with the heat capacity of the stored materials, will entirely suppress the daily temperature cycle. A lower U value risks accumulating heat from lighting and equipment.
The long period temperature stabiliser is the floor and the ground beneath it. A computer prediction for the coldest and the warmest months in Ribe, Denmark, is shown in figure 2.
The ground under a building of reasonable size for a museum store behaves thermally as part of the building, after a year or two of acclimatisation. The heat flow is negligible below 3 m, so there is no need for insulation at depth. This simple construction gives a smooth annual cycle with an amplitude about half the span of the monthly averages outside. Figure 3 shows the measured temperature gradients in the ground underneath the museum store in Ribe, compared with the open field beside it. The upper graph continues the measurement up through the building. The vertical temperature span within the store never exceeds two degrees.
Since only dehumidification is installed, the winter temperature must be designed to keep the RH moderate as outside air slowly infiltrates. In Europe this means a February temperature approximately 7°C above ambient. This will keep the interior at about 50% RH. The wall insulation thickness should be calculated to ensure this temperature difference. Figure 4 shows a measured example of such a floating temperature regime, from the cool temperate climate of Ribe.
One notices that there is dehumidification continuing during the period shaded B while at the same time the outside air has a lower water vapour content than that inside. During this period it would be more efficient simply to pump outside air in, rather than dehumidify recirculated inside air. The effectiveness of pumping is well displayed in the record from the small archive of the Arnamagnæan Institute of Copenhagen University, which has no dehumidifier; instead, it pumps in outside air when the vapour content will push the inside RH towards the target 50% (Padfield et al. 2018).
Condensation dehumidifiers use around 1 kWh per kg of water collected. At 10°C and 50% RH the condensing surface must be below zero and thus be covered with ice. There has to be intermittent defrosting of the cold surface, but such a brief interruption is entirely smoothed out by humidity buffering by the stored materials.
Absorption dehumidifiers operate at around 2 - 3 kWh/kg but can reach a lower RH. The measured energy consumption of absorption dehumidifiers in real conditions in several museum stores has been reported by Ræder Knudsen and Lundbye (Knudsen and Lundbye 2017).
Recently designed stores use less than one kWh/m3 per year. In comparison, an archive controlled by air conditioning uses about 25 kWh/m3 per year (Ryhl-Svendsen et al. 2010).
Pumping outside air when it is of suitable vapour content is the most efficient form of RH control, but for long periods the weather prevents it, so mechanical dehumidification cannot be avoided.
Both humidity and temperature buffering rely on minimal exchange of heat and moisture with the outside air. Ventilation is often advocated to hinder mould growth. It works by forcing a uniform temperature. However, in a purpose-built store the insulation alone will ensure a uniform temperature. Furthermore, the temperature difference between inside and outside will be smaller than in a dwelling.
In a store with only occasional human presence, there is no need for 'fresh' air, together with the outdoor pollutants it entrains. Museum stores cannot completely avoid using modern materials which outgas volatile substances, also the stored objects will contribute their own volatile components. Therefore it is sensible to consider installing a pollutant filter in the recirculating dehumidification system. Both absorption (Wolfrum, Peterson, and Kozubal 2008) and condensation dehumidifiers remove condensable pollutants from the air, so the decision to add a separate absorber needs scientific confirmation. At the winter temperature minimum, the production, diffusion and reaction of internally generated pollutants is low (Ryhl-Svendsen et al. 2012) (Ryhl-Svendsen, Jensen, and Larsen 2014).
Because of the lack of reliable predictive calculations, designers deliberately ignore the influence of the stored objects on their micro-climate and thus provide far too powerful mechanical control. However, for archives in particular, buffering by the paper provides an enormous stability to the RH. Padfield and Jensen (Padfield and Jensen 2011) attempted to remedy this fear of the uncertain reliability of involving the stored materials at the building design stage. They proposed a simple way to estimate the buffer capacity of an unbuilt storage space. They attribute to each stored item, a box of papers for example, a buffer value (B-value) which is equal to the volume of space whose RH would change by exactly the same amount as the equilibrium RH change of the box when subjected to the same addition of water. The B-values of each box in the store add up to give a "virtual volume" for the store, which is many times greater than its actual volume, several hundred times in the case of a well-filled paper archive. Infiltrating water vapour disperses into this virtual volume, thus giving a much smaller change in RH.
The powerful influence of humidity buffering is well displayed by the performance of the Suffolk Record Office in Ipswich, UK. In this building there is no mechanical dehumidification to interfere with the natural progress of the indoor climate. The annual average RH within is kept below the annual average outside by winter heating alone. The extent of the disequilibrium between the interior and the air outside is shown by the difference in the mixing ratio, shown at the bottom of figure 5. The summer mixing ratio outside is nearly always higher than that inside, meaning that infiltration will tend to increase the indoor RH, by adding to the water vapour concentration. This only happens to a modest extent.
Another archive that operates in a similar way is the Arnamagnæan archive of Copenhagen University (Padfield et al. 2018). It does not even have a thermostat to control its RH, which is maintained by a balance between heat flow from the warm interior of the office building and the heat flow through two outside walls. Fine control is however achieved by pumping.
Winter warming uses more energy than dehumidification in a new building with a low air exchange rate, but it may be a useful solution for busy archives. A chart comparing energy use by the techniques described in this article is given by Larsen (Larsen 2018).
Be warned that active RH control by heating to correct a too high RH (commonly called "Conservation Heating") is catastrophic in a tightly packed store with low air exchange, because raising the temperature will raise the RH (Padfield 1996).
As environmental standards become less rigid, reliance on orthodox air conditioning continues. One reaches a point where air conditioning is used when the same specification can be attained without it. An example is the Pierrefitte archive in Paris (Bonandrini 2017). This is fully air conditioned to a specification of 16° - 24°C and 40% - 57% RH. However, its performance is so close to that of the Suffolk archive that a very small change of operating point, lowering the target RH to 40%, would make air conditioning unnecessary (Padfield 2017).
The Suffolk archive is now air conditioned, because its upper room occasionally got warmer than permitted by a standard that has now been superseded. the Pierrefitte archive is air conditioned because it was explicitly designed to hold its specification in an empty building, destined to be filled with archived material.
Standards have become less strict over the last decade but still specify limits which take no account of the outside climate. The British Standard PAS198 (BSI 2012) attempted to introduce the concept that the curator should be given the information to make her own judgement about storage conditions. However, this document illogically fell back on giving exact numerical recommendations for temperature and RH.
Environmental conservators comprise the only group of conservation specialists which is constrained by arbitrary standards whose authority is based on the prestige of the issuing institution rather than on closely argued, well documented, and peer reviewed science (Ashley-Smith 2016). Nevertheless standards exert huge influence over the cost and complexity of museum buildings. We must modernise the process of evolving standards, in the form of an open wiki-style process, hosted by a conservation institution rather than by a general standards institution.
There is now abundant evidence from measured buildings, for the effectiveness and cheapness of low energy air conditioning for museum stores and archives. The preferred option is to allow the temperature to drift freely in a gentle cycle centred on, but with a span well within, the outdoor temperature cycle. This diminished annual cycle is enforced by the thermal inertia of the ground beneath the building, supplemented by the heat capacity of the stored materials and of the building itself, and by thermal insulation of the walls and roof. In the temperate zone, summer dehumidification will be needed. This climate control consumes less than a tenth of the energy required by air conditioning.
Some busy archives will choose to run at a temperature nearer to the human comfort zone. This also can be achieved relatively cheaply by winter heating to approximately 15°C, combined with humidity buffering by the archived papers and by good airtightness. If the internal buffering is not quite adequate, the peak RH can be moderated by intermittent pumping of outside air when, by chance of the weather, it has a water content that will tend to reduce the interior RH towards its target.
Current environmental standards do not make it easy to collaborate with the weather. Arbitrary limits to temperature, particularly, are set without consideration that occasional, or even permanent, excursions beyond the set span will not cause detectable damage to artefacts, yet allow much simpler, cheaper and more reliable climate control.
Climate data for the Suffolk Record Office are from Dominic Wall. Climate data from the Pierrefitte archive are from Bruno Bonandrini. An extended version of this article is available at (Padfield et al. 2017). Many relevant, detailed articles are on the conservationphysics.org website.
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Tim Padfield, email@example.com, is an independent conservation scientist based in Roskilde, Denmark. He has a MA in chemistry from Oxford University and a PhD in building physics from the Technical University of Denmark. His publications are at www.conservationphysics.org
Morten Ryhl-Svendsen has a PhD in conservation-restoration from the School of Conservation in Copenhagen. He worked for 16 years at the National Museum of Denmark, where he was a senior researcher in preventive conservation. Since 2014, he has been associate professor at the Danish School of Conservation. His research area is the environment's impact on materials, with a special interest in air quality and pollution. firstname.lastname@example.org
Poul Klenz Larsen has a master degree (1990) and a PhD (1998) in building physics from the Technical University of Denmark. He is a senior consultant for historic buildings at the National Museum of Denmark. His main working areas are energy efficient heating and climate control, salt and moisture in historic masonry structures and traditional mortars for restoration. email@example.com
Lars Aasbjerg Jensen has a MA from The School of Conservation of the Danish Academy of Art. He works at the National Museum of Denmark, specialising in climate measurement and control in exhibitions and storage spaces. firstname.lastname@example.org
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