Volume 27, number 1, June 2013

Key measures for saving energy in schools

In recent years, many energy efficiency technologies and strategies have made their appearance. Despite the great variety of new technologies and strategies, the measures discussed in this article are solutions adopted in several projects recognized by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) and the Association of Energy Engineers (AEE) and are therefore preferred solutions.

Condensing boiler combined with a low-temperature hydronic system and high ΔT

Heating by a hot water system has many advantages compared with alternative heating solutions. Thermal comfort, durability, and power recovery following a drop in temperature are some examples that explain why such a system is installed in the majority of commercial and institutional buildings. Now, the design of traditional hydronic heating systems is based on high temperature supply at 71°C – 82°C (160oF – 180oF) with a differential of 11°C (20oF) between supply and return to the boiler. Since new construction standards and methods are now stricter and buildings are more and more efficient, it thus becomes worth choosing systems with a higher temperature differential (ΔT = 22°C or 40oF) that permit operating at a much lower temperature. In their article published in the ASHRAE Journal,1 Ryan Thorson and David Williams mentioned some very efficient designs that use system temperatures of 60oC supply and 38°C return (140°F and 100oF). At these temperatures, the efficiency of the condensing boilers is maximized to reach an efficiency rating of over 95%.

In certain cases, sizing at low temperatures requires a larger heat exchange surface. So, while at first sight, this solution might seem more expensive since it requires larger exchange surfaces and therefore larger coils and convectors, selecting a high ΔT design means that the size of the piping and the pumping forces can be reduced significantly. Energy transmission through the transfer fluid is described in the following equation.2

Q energy transmission = Flow x ΔT x Conversion Factor1

If the temperature differential is increased by a factor of 2,
then the flow can be reduced by the same factor and the motive force cubed (23= 8).

The sizing of new systems is detailed in an article published in the Informa-TECH bulletin in February 2010. In projects to modernize boiler plants, providing for variable speed drives on the pump motors is advantageous in order to maximize the potential of the new boilers, i.e. a high ΔT in mid-season and then, in very cold weather, return to the flows and temperatures in the original designs for the heaters with a much lower temperature differential.

Displacement ventilation

Diffusers are often overlooked when space is being remodelled. However, a quick energy analysis shows that the distribution strategy will have a major energy impact on the system’s efficiency. Selecting displacement diffusion can help achieve ventilation efficiencies in the order of 1.0 – 1.2 for heating or air conditioning. A ceiling ventilation system with traditional diffusers gives a distribution efficiency of about 0.8 during the heating period.3 Displacement ventilation thus permits a gain of 25% – 50% (1.0 – 1.2/0.80). This is a major impact if costs are analyzed over the lifecycle of the whole system. For example, the size of the ventilation ducts may be reduced, as well as air flows (total and fresh air).

Management of fresh air intake and heat recovery

The intake of fresh air is an essential element in comfort and it is one of the largest energy consumers. Heat recovery is therefore essential for energy efficiency. There are various types of equipment, each with its own advantages and disadvantages, depending on the particularities of the building. For systems that are simple to operate and easy to maintain, “passive” units (without compressors) are the preferred options. In certain cases, less efficient exchangers, such plate heat exchangers, may be selected when the weather is very cold. Although there are also more efficient units that recover more heat, units that do not create condensation do not require frost controls and so can be viable alternatives with good potential for heat recovery when construction and maintenance budgets are limited.

Control of air flow

As indicated above, ventilation accounts for a large part of a building’s energy costs. The installation of variable flow systems with VAV (variable air volume) boxes is thus a frequently used energy saving strategy in new installations. However, several technical references from ASRHRAE and the AEE warn us not to neglect quality control and readings of air flow as well as readings of air quality (CO2). In fact, the efficiency of a system can deteriorate rapidly without monitoring tools.

A loss of uncontrolled flow in one area or branch will create a climate fight (heating air-conditioned air). This will lead to over-consumption to cool heated air as well as have a volume impact on the ventilation system’s motive force. Control components will thus facilitate better management and avoid energy losses from the over-ventilation of low-load zones.

The impact of operating costs for ventilation can be observed by comparing the efficiency rating of recent buildings, mostly ventilated, against older, often non-ventilated buildings. While we tend to think that more recent buildings, with good envelopes and built according to efficient construction methods, require less energy than old buildings, the latter generally do better in terms of overall energy ratings, namely 3% Р10% on average in Qu̩bec, and more than 20% according to a detailed study by the AEE of four American schools.4

In short, we can see that the traditional measures described in this article define the basics and the methods to be followed in designing buildings that are both efficient and durable.

Gaz Métro supports the implementation of improvement projects through its energy efficiency financial assistance programs. Between 2007 and 2013, Gaz Métro granted more than $5.1 M in financial assistance to Québec school commissions, thus reducing their operating costs and saving more than 9.5 million m3 of natural gas, representing 18,124 tonnes of CO2.

School commissions: Breakdown of savings (m3) subsidized by Gaz Métro – 2007-2012

Mathieu Rondeau, Eng., CEM, LEED GA®
DATECH Group, Gaz Métro

1 Ryan Thorson and David Williams, “Old School Learns Cool New Tricks,” Vol. 54, No. 5, May 2012.

2 Pump calculation: for pumping power “P” based on flow “D”: P 1/P2 = (D1/ D2)3.

3 Standard 62.1-2010, “Ventilation for Acceptable Indoor Air Quality,” ASHRAE, 2010.

4 Dr. Wayne C. Turner, “Energy Engineering,” AEE Journal, Vol. 109, No. 6 2012.

Note: According to data published in the energy report for all school commissions in Qu̩bec, 2010-2011, for the same type of buildings, ventilated buildings consumed, on average, between 3% Р10% more energy than non-ventilated buildings.