A Plug for HVAC Savings from Efficient Lab Equipment

We recently worked with our Center for Energy Efficient Labs partners My Green Lab and Frontier Energy on an Emerging Technologies study funded jointly by the three California IOUs. The study involved testing the energy performance of modern, high-efficiency ultra-low temperature (ULT; -80C) lab freezers and calculating the energy savings achievable via the use of high-efficiency models. The savings opportunities are significant: high-efficiency ULTs use less than half the energy of inefficient ones, and these savings can amount to more than 500W per freezer.

If we start replacing our ULT freezers with efficient ones, do we get extra bonus HVAC savings or do we get penalties? We needed to find out.

But freezers don’t exist in isolation: lab buildings are complex, interactive systems. If we start replacing our ULT freezers with efficient ones, do we get extra bonus HVAC savings or do we get penalties? We needed to find out.

We all know someone who insists that incandescent light bulbs are best because they provide lots of free heat along with the light. But we also know to ask that friend what happens in summer when the AC is on and you don’t want all that extra heat.

We hear the same level of certainty (though in the opposite direction) when we’re being sold energy-efficient appliances: we’re told that reducing the plug load will reduce heat dissipation and so HVAC cooling loads will be lower and we’ll get bonus energy savings of perhaps 30% of the “direct” savings from the appliance itself. But this isn’t always true either. And in labs, where everything is more interesting, it’s often not the case.

To be clear up front: regardless of the HVAC impact, the more efficient appliance will (almost) always result in overall energy savings; it’s just a question of how much. In situations commonly found in labs, the total – direct plus HVAC – energy cost savings from using an efficient appliance can differ by a factor of 2 depending on where the appliance sits. If you’re targeting an ROI, the HVAC impact matters.

So why does it matter so much where you put your lab appliance? As HVAC environments go, labs are special and also quite varied. For safety reasons, labs are typically conditioned using 100% outside air and are ventilated continuously and often excessively (and expensively). Labs also tend to have higher equipment loads than other kinds of space – but not always high enough to make up for all the cold air being delivered by the air handlers. We’ll skip discussing plenty of important details involving HVAC system types, control strategies, space loads and so on and just make the following statements, which are generally true:

  • In dedicated equipment rooms, plug loads are high and so reduced heat dissipation usually leads to reduced airflow and to bonus HVAC savings. In the best-case scenario, removing plug loads from a critical “rogue zone[1]” could allow AHU supply air temperature to be reset upwards, producing overall system-wide savings far exceeding the direct appliance savings.
  • In open labs, plug loads are often quite low (less than 1 W/sf in many cases) and at typical ventilation rates the space is often almost always in reheat mode. Reducing plug loads then means that more reheat is needed and the HVAC impact is an energy penalty. Provided your reheat is not electric resistance (and I hope it’s not), the overall energy cost savings are still positive – just lower – and therefore your retrofit is less cost effective.

  • Other scenarios can also occur. If the cooling system is loaded beyond capacity (as is common in packed hallways not originally designed to house lab equipment), or if warm air leaving plug loads is directly exhausted from the space without mixing with room air, changing the efficiency of the appliances probably won’t affect the HVAC energy consumption.[2]


So How Big Are the Interactive Savings for ULT Freezers?

First question: where are the freezers located?
We asked scientists and facility managers (more than 400 people in all) where their ULTs sit. We found that about 60% are in spaces designed for high equipment loads (where efficient freezers lead to HVAC savings).

Second question: how big are the HVAC savings impacts for typical lab buildings in California?
To answer that, we need to know what a typical lab building looks like. We surveyed facility managers and used our extensive record of energy audits to establish typical HVAC system types, control strategies, and ventilation rates for existing lab buildings and common space types within them. We then constructed a building energy model of the typical California laboratory and simulated a range of plug loads for each space type in which freezers are found. The results (HVAC cost savings for Oakland weather) are summarized in the figure below.

HVAC energy savings in labs

The total (direct plus HVAC) energy cost savings therefore vary by a factor of 1.6, just depending on the freezer’s location in the building.

Combining these results with the location distribution of freezers shown in the pie chart, we found that the average HVAC cost savings from replacing an inefficient ULT freezer with an efficient one are just 10% of the direct freezer savings. (Based on Oakland weather; numbers vary a bit with climate.)

So the overall average HVAC impacts are quite a bit smaller than an overconfident salesperson might claim. These findings are useful when developing policies and incentive programs, but there are some more directly actionable lessons too:

  • Always consider buying high-efficiency ULT freezers (see the ENERGY STAR website for a list of approved freezers, and check for rebates with your local utility company) but remember that your total energy savings will depend on where you put the freezer!
  • If you’re replacing a freezer and want to maximize total energy cost savings, choose to replace one that’s in a crowded equipment room.
  • If you’re adding a new freezer and want to have the lowest total energy cost impact, put it in a reheat-dominated open lab.

These rules are only general strategies for the typical facility, based on common HVAC systems and operation. If you want to find the best way to arrange your freezers, or if you’re interested in finding control strategies to make your whole lab HVAC system operate more efficiently, we’d love to help.

Newsflash: The Center for Energy Efficient Labs team is now studying high-efficiency -20C lab freezers! Be on the lookout for our industry survey (coming soon)…

[1] A rogue zone is one that drives up the energy use of a whole system by constantly calling for heating or cooling, preventing reset control strategies from working as intended.

[2] Freezers are a slightly special kind of appliance: they don’t just dissipate heat but also move it around. If the freezer’s rejected heat is directly exhausted, the freezer may have a net cooling effect on the space. The HVAC energy impact of a more efficient freezer then depends on whether the efficiency was achieved by reducing the freezer cooling load or improving its refrigeration efficiency. And the interactive effects go in the other direction too: making a space slightly less overloaded won’t change the HVAC energy consumption, but it might allow the room temperature to drop a little, which then gives the freezers an extra efficiency boost.


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