Energy saving

This section explains the connection between the environmental recommendations and saving energy. It clarifies the message that the wider permitted variation is intended to allow an annual cycle rather than condone large day to day variation. The large allowed range is based on quantitative experiments with freshly made (by museum standards) items and on observations of damage to ancient items. Museum specimens have an infinite variety of weaknesses in the form of degraded and contaminated materials, and weakened joints between them. There is still good justification for the precautionary principle, because accurate estimation of the vulnerability of a particular item to environmental insult will never be practical. In this document we introduce the 'energy efficient precautionary principle' where the wide limits sanctioned by lack of firm evidence of harm are supplemented by a target range for the environment in new build or in deep renovation of buildings, which is much narrower, but attainable with current construction technology without any increase in energy consumption.

Consequences of a very narrow environment specification

A very tight specification (±1°C) means that air conditioning will pump air through ducts at less than the intended temperature and relative humidity, so that supplementary devices close to each individual room can warm and humidify the air to the exact specification. This alone increases the energy use but if neighbouring rooms have slightly different error signals from their sensors, it is also possible that one air conditioning zone will be acting in opposition to the neighbouring zone.

Consequences of a narrow environment specification

A narrowly controlled climate (±3°C) will seldom be the same as the outside air, so energy is used to adjust the incoming air to the specification. A narrow RH range impedes economising on energy by only pumping in air when, by chance, it has a suitable water vapour content (enthalpy control in HVAC jargon).

Consequences of a constant set point far from the outside temperature and water vapour concentration

Energy must be used to change the air temperature, and then usually energy is necessary to adjust the RH. This is mostly a winter expense in a cool temperate climate when the inside temperature is set to about 22°C. Not only must the temperature be raised but also the relative humidity, since outside air contains too little water vapour.

Consequences of a temperature and RH varying in an annual cycle

A lower winter temperature reduces the heating energy needed and also reduces the humidification demand. A lower winter RH will make humidification rarely necessary.

Choosing an energy efficient environment specification for storage

The most economical temperature control is none - allowing the indoor temperature to follow that outside, moderated by the thermal inertia of the building. The annual average temperature will be almost the same inside as out. In practice it is possible to keep the annual variation within a ten degree range. In stores, there is no need for fresh air for people to breathe. However, it is difficult to eliminate leaks, so dehumidification will be necessary, mostly in summer. The low air exchange rate will make this process a small consumer of energy and the dominantly summer demand, combined with humidity buffering, allows intermittent solar energy to be used. If air filtration is needed, to absorb internally generated pollutants, there will be some fan energy released into the room. The annual average will be a few degrees above the outdoor average, giving some dehumidification, so the total energy consumption will still be small.

Energy use in cold stores

In cold stores below -5°C the temperature is nearly always below ambient so the energy consumption will be proportional to the difference between the 10°C annual average outdoors and the inside temperature. As with cool storage, the air exchange rate is slow, so the energy cost of humidity control (always dehumidification) will be small.

Energy for lighting

Historically, incandescent lamps have been favoured by exhibition designers. The small source and the possibility of focussing to give well defined shadows made these lamps preferable to fluorescent tubes. Incandescent lamps have an efficiency of 25 lumens per watt at best. Compact source light emitting diodes with similar light characteristics have an efficiency over 100 lm/W and increasing all the time because of intense research. It is now possible to illuminate an exhibition using about 10 W/m². Maximising the fraction of generated light which falls on the exhibits is an important action in saving energy for climate control of exhibitions.

The magnitude of energy saving

The quantitative benefits of widening the allowed range of temperature and RH on an annual cycle can be evaluated on this page:
http://www.conservationphysics.org/atmcalc/energyusecalc.php
This program calculates the raw energy use, without the aid of heat and moisture exchangers. Clever engineering can reduce energy use but there are fundamental physical limits to how little energy is needed to move air from the outside conditions to the desired indoor climate. The closer the inside approaches the outside temperature, the less the power needed.